The kings of Leinster () ruled from the establishment of Leinster during the Irish Iron Age until the 17th century Early Modern Ireland. According to Gaelic traditional history found in works such as the Book of Invasions, Leinster was created during the division of Ireland among the Irish Gaels, descendants of Milesius: Leinster was one of the territories held by the offspring of Heremon. In the 7th century BC, the branch of the Heremonians who would establish Leinster, starting with Úgaine Mór, were also High Kings of Ireland and Kings of Tara. Their ascent to hegemony in Ireland was associated with the decline in influence of their Ulster-based Heremonian kinsmen from the Érainn.
Apart from Úgaine Mór, other prominent Kings of Leinster from this period who were also High Kings of Ireland were Labraid Loingsech and Cathair Mór. A mythology developed that Labraid Loingsech had horses ears: he spent some time exiled in Transalpine Gaul (dated roughly to the period of the Roman–Gallic wars) where his grandmother was from and returned to Ireland with Gaullish mercenaries. He established a base in the area, which was renamed from "Gailian" to Leinster, in reference to the pointed-spears held by the Gaullish Gaesatae mercenaries who provided the backbone of Labraid Loingsech's powerbase and brought him to the High Kingship. Cathair Mór, who was also a High King of Ireland, is perhaps the most important figure genealogically in Leinster as all of the subsequent kinship groups which ruled Gaelic Leinster claimed descent and legitimacy to rule from one of his ten sons who had offspring.
By the time of Early Christian Ireland, the Laighín had lost their grip on Irish hegemony with the rise of the descendants of Conn of the Hundred Battles (the Connachta and the Uí Néill). The latter pushed down into Leinster and created the Kingdom of Meath based at Uisnech, under the Southern Uí Néill from territories belonging to the Kingdom of Leinster and to the west Osraige was taken from the Laighín by the Corcu Loígde of Munster.
During the 5th to the early 8th centuries, the Kingship of Leinster was contested by various different branches of the Laighín, including the Uí Cheinnselaig (ancestors of the Mac Murchada and Caomhánach), the Uí Dúnlainge (ancestors of the O'Byrnes and the O'Tooles), the Uí Bairrche (ancestors of the Mac Gormáin), the Uí Máil (ancestors of the Ó Conchobhair Uí Failghe) and others.
From the 8th until the 11th century, the kingship of Leinster rotated in a remarkably stable arrangement between three branches of the North Leinster Uí Dúnlainge kinship, namely the Uí Fáeláin (ancestors of the O'Byrnes), the Uí Muiredaig (ancestors of the O'Tooles) and the Uí Dúnchada (whose descendants became the MacGillaMoCholmóc and later the FitzDermots). In the 9th century, the Laighín also regained control of Osraige but it remained a largely independent realm under the Mac Giolla Phádrag, from the semi-autonomous Uí Failghe kinship group.
The Uí Cheinnselaig in South Leinster took back control of the kingship of Leinster in the 11th century, with Diarmait mac Máel na mBó, who became the first Leinsterman to be High King of Ireland in over a thousand years and claimed to the most senior line of the Laighín. An enemy of the Vikings of Dublin, he gave refuge to the sons of Harold Godwinson after the Normans conquered England. It is from this line that the Mac Murchada family originate and later Diarmait Mac Murchada would be implicated in the 12th century Norman invasion of Ireland after he tried to win back his Leinster throne. The reigning dynasty adopted the surname Caomhánach (Kavanagh) and continued to rule a rump Kingdom of Leinster until the early 17th century, with the last recorded King of Leinster being Domhnall Spáinneach Mac Murchadha Caomhánach. Throughout the centuries after the Norman invasion, several Kings and also leaders from dynasties who had previously held the Kingship of Leinster, continued to resist the invasion and hounded the English Pale periodically from the Wicklow Mountains: including the leaders of the O'Toole and O'Byrne clans (notable examples include Art Óg Mac Murchadha Caomhánach and Fiach McHugh O'Byrne) until the 17th century.
Legendary Kings Mesgegra
Kings of Leinster
Laigin, classical antiquity
| Úgaine Mór634–594 BC || || Son of Eochu Buadach||unknown || 594 BCE |- | Lóegaire Lorc594–592 BC || || Son of Úgaine Mór||unknown || 592 BCE |- | Ailill Áine592–592 BC || || Son of Lóegaire Lorc||unknown || 592 BCE |- | Cobthach Cóel Breg592–542 BC || || Son of Úgaine Mór||unknown || 542 BCE |- | Labraid Loingsech542–523 BC || || Son of Ailill Áine mac Lóegaire Lorc||unknown || 523 BCE |- | Cathair Mór 119–122 AD || || Son of Fedlimid Fir Urglais mac Cormaic Gealtach||11 children || 122 AD
Uí Cheinnselaig, 5th century
| Fiacha Baicheda mac Cathair Mór – || || Son of Cathair Mór ||unknown || 416 |- | Bressal Bélach mac Fiacha Baicheda –436 || || Son of Fiacha Baicheda mac Cathair Mór ||unknown || 436 |- | Énnae Cennsalach mac Labhradh – || || Son of Labhradh mac Bressal Bélach ||unknown || unknown
Uí Bairrche, 5th century
| Muiredach Mo Sníthech mac Dáire Barrach –(claimant) || || Son of Dáire Barrach mac Cathair Mór ||unknown || unknown |- | Móenach mac Muiredach Mo Sníthech –(claimant) || || Son of Muiredach Mo Sníthech mac Dáire Barrach ||unknown || unknown
Uí Enechglaiss, –446
| Mac Cairthinn mac Coelboth –446 || unknown || Son of Cóelbad ||unknown || 446
Uí Cheinnselaig, 446–483
| Crimthann mac Énnai 446–483 || || Son of Énnae Cennsalach ||unknown || 483
Uí Garrchon, 483–495
| Fincath mac Garrchu 483–485 || unknown || Son of Garrchú mac Fothaid ||unknown || 485 |- | Fráech mac Finchada 485–495 || unknown || Son of Fincath mac Garrchu ||unknown || 495
Uí Dúnlainge, 495–592
| Ailill mac Dúnlainge 495–527 || || Son of Dúnlaing mac Énda Niada ||Etromma Ui Tellain || 527 |- | Cormac mac Ailillo527–535 || || Son of Ailill mac Dúnlainge ||unknown || 535 |- | Coirpre mac Cormaic535–546 || || Son of Cormac mac Ailillo ||unknown || 546 |- | Colmán Már mac Coirpre546–576 || || Son of Coirpre mac Cormaic ||unknown || 576 |- | Áed Dub mac Colmáin576–592 || || Son of Colmán Már mac Coirpre ||unknown || 639
Uí Máil, 592–595
| Áed Dibchine 592–595 || || Son of Seanach Diodhach mac Carthann Muadh ||unknown || 595
Uí Cheinnselaig, 595–624
| Brandub mac Echach 595–605 || || Son of Echu mac Muiredaig ||unknown || 605 |- | Rónán mac Colmáin 605–624 || || Son of Colmán mac Cormaic ||unknown || 624
Uí Máil, 624–633
| Crimthann mac Áedo 624–633 || || Son of Áed Dibchine ||unknown || 633
Uí Dúnlainge, 633–666
| Fáelán mac Colmáin 633–656656–666 || || Son of Colmán Már mac Coirpri ||unknown || 666
Uí Cheinnselaig, 656
| Crundmáel Erbuilc 656 || || Son of Rónán mac Colmáin ||unknown || 656
Uí Máil, 666–680
| Fiannamail mac Máele Tuile 666–680 || || Son of Máele Tuile mac Rónán Crach ||unknown || 680
Uí Dúnlainge, 680–693
| Bran Mut mac Conaill 680–693 || || Son of Conall mac Fáelán ||unknown || 693
Uí Máil, 693–715
| Cellach Cualann 693–715 || || Son of Gertighe mac Diocolla Dana ||unknown || 715
Uí Dúnlainge, 715–728
| Murchad mac Brain Mut 715–727 || || Son of Bran Mut mac Conaill ||unknown || 727 |- | Dúnchad mac Murchada 727–728 || || Son of Murchad mac Brain Mut ||unknown || 728
Uí Fáeláin (Uí Dúnlainge), 728–738
| Fáelán mac Murchada 728–738 || || Son of Murchad mac Brain Mut ||unknown || 738
Uí Dúnlainge, 738
| Bran Becc mac Murchada 738 || || Son of Murchad mac Brain Mut ||unknown || 738
Uí Cheinnselaig, 738
| Áed mac Colggen 738 || || Son of Colcú mac Bressail ||unknown || 738
Uí Muiredaig (Uí Dúnlainge), 738–760
| Muiredach mac Murchada 738–760 || || Son of Murchad mac Brain Mut ||unknown || 760
Uí Dúnchada (Uí Dúnlainge), 760–776
| Cellach mac Dúnchada 760–776 || unknown || Son of Dúnchad mac Murchada ||unknown || 776
Uí Fáeláin (Uí Dúnlainge), 776–785
| Ruaidrí mac Fáeláin 776–785 || || Son of Fáelán mac Murchada ||unknown || 785
Uí Muiredaig (Uí Dúnlainge), 785–795
| Bran Ardchenn 785–795 || || Son of Muiredach mac Murchada ||unknown || 795
Uí Dúnchada (Uí Dúnlainge), 795–808
| Fínsnechta Cethardec 795–808 || unknown || Son of Cellach mac Dúnchada ||unknown || 808
Uí Muiredaig (Uí Dúnlainge), 808–818
| Muiredach mac Brain 808–818 || || Son of Bran Ardchenn ||unknown || 818
Uí Fáeláin (Uí Dúnlainge), 818–829
| Muiredach mac Ruadrach 818–829 || || Son of Ruaidrí mac Fáeláin ||unknown || 829
Uí Muiredaig (Uí Dúnlainge), 829–834
| Cellach mac Brain 829–834 || || Son of Bran Ardchenn ||unknown || 834
Uí Dúnchada (Uí Dúnlainge), 834–838
| Bran mac Fáeláin 834–838 || unknown || Son of Fáelán mac Cellaig ||unknown || 838
Uí Muiredaig (Uí Dúnlainge), 838–854
| Lorcán mac Cellaig838–851 || || Son of Cellach mac Brain ||unknown || unknown |- | Túathal mac Máele-Brigte 851–854 || || Son of Muiredach mac Brain ||unknown || 854
Uí Dúnchada (Uí Dúnlainge), 854–862
| Ruarc mac Brain 854–862 || unknown || Son of Bran mac Fáeláin ||unknown || 862
Uí Fáeláin (Uí Dúnlainge), 862–863
| Muirecán mac Diarmata 862–863 || || Son of Diarmata mac Ruadrach ||unknown || 863
Uí Muiredaig (Uí Dúnlainge), 863–871
| Dúnlaing mac Muiredaig863–869 || || Son of Muiredach mac Brain ||unknown || 869 |- | Ailill mac Dúnlainge 869–871 || || Son of Dúnlaing mac Muiredaig ||unknown || 871
Uí Fáeláin (Uí Dúnlainge), 871–884
| Domnall mac Muirecáin 871–884 || || Son of Muirecán mac Diarmata ||unknown || 884
Uí Dúnchada (Uí Dúnlainge), 884–885
| Muiredach mac Brain 884–885 || unknown || Son of Bran mac Fáeláin ||unknown || 885
Uí Fáeláin (Uí Dúnlainge), 885–909
| Cerball mac Muirecáin 885–909 || || Son of Muirecán mac Diarmata ||unknown || 909
Uí Muiredaig (Uí Dúnlainge), 909–917
| Augaire mac Aililla 909–917 || || Son of Ailill mac Dúnlainge ||unknown || 917
Uí Dúnchada (Uí Dúnlainge), 917–943
| Faelan mac Muiredach 917–942 || unknown || Son of Muiredach mac Brain ||unknown || 942 |- | Lorcán mac Fáelán 942–943 || unknown || Son of Faelan mac Muiredach ||unknown || 943
Uí Fáeláin (Uí Dúnlainge), 943–947
| Bran Fionn mac Máel Mórda 943–947 || || Son of Máel Mórda mac Muirecain ||unknown || 947
Uí Muiredaig (Uí Dúnlainge), 947–958
| Túathal mac Úgaire 947–958 || || Son of Augaire mac Aililla ||unknown || 958
Uí Dúnchada (Uí Dúnlainge), 958–966
| Cellach mac Faelan 958–966 || unknown || Son of Faelan mac Muiredach ||unknown || 966
Uí Fáeláin (Uí Dúnlainge), 966–972
| Murchad mac Brain Fionn 966–972 || || Son of Bran Fionn mac Máel Mórda ||unknown || 972
Uí Muiredaig (Uí Dúnlainge), 972–978
| Úgaire mac Túathail 972–978 || || Son of Túathal mac Úgaire ||unknown || 978
Uí Dúnchada (Uí Dúnlainge), 978–1003
| Domnall Claen 978–984 || unknown || Son of Lorcán mac Fáelán ||unknown || 984 |- | Donnchad mac Domnall Claen 984–1003 || unknown || Son of Domnall Claen ||unknown || unknown
Uí Fáeláin (Uí Dúnlainge), 1003–1014
| Máel Mórda mac Murchada 1003–1014 || || Son of Murchad mac Brain Fionn ||unknown || 1014
Uí Muiredaig (Uí Dúnlainge), 1014–1016
| Dúnlaing mac Tuathal 1014 || || Son of Túathal mac Úgaire ||unknown || 1014 |- | Donncuan mac Dúnlainge 1014–1016 || || Son of Dúnlaing mac Tuathal ||unknown || 1016
Uí Fáeláin (Uí Dúnlainge), 1016–1018
| Bran mac Máel Mórda 1016–1018 || || Son of Máel Mórda mac Murchada ||unknown || 1052
Uí Muiredaig (Uí Dúnlainge), 1018–1033
| Augaire mac Dúnlainge 1018–1024 || || Son of Dúnlaing mac Tuathal ||unknown || 1024 |- | Donnchad mac Dúnlainge 1024–1033 || || Son of Dúnlaing mac Tuathal ||unknown || unknown
Mac Giolla Phádraig (Dál Birn), 1033–1039
| Donnchad mac Gilla Pátraic 1033–1039 || || Son of Gilla Pátraic mac Donnchada ||unknown || 1039
Uí Muiredaig (Uí Dúnlainge), 1039–1042
| Murchad mac Dúnlainge 1039–1042 || || Son of Dúnlaing mac Tuathal ||unknown || 1042
Mac Murchada (Uí Cheinnselaig), 1042–1115