It is unclear whether a form of Early Irish astrology existed prior to contact with Western astrology, as the earliest Irish language sources are simply translations from standard Western sources. Historian Peter Berresford Ellis argues that although there is evidence of the development of Irish astrology from the 7th century AD onwards, anything earlier is left to conjecture based on continental Celtic artifacts like the Coligny calendar and reconstructions from historical documents.
While the pre-Celtic megaliths in Ireland are often aligned with solar and lunar phenomena, no evidence has been found for the type of planetary symbol system as seen in other cultures' systems of astrology.
Before Christianity Peter Berresford Ellis has speculated on possible Indo-European connections between pre-Christian Irish astrology and Hindu astrology. The comparative research between the two astrological systems and their similarities, as well as their differences from Babylonian astrology from which modern Western astrology derives does not only concern these two systems alone. It implies the Proto-Indo-Europeans had a native system of astrology, which in turn, implies a native Indo-European cosmology which survived in these two cultures. He cites the works of Aibhistín, a 7th-century Irish astrologer as tentative evidence of a native system of lunar mansions similar to the nakshatras in Hindu astrology. Ellis has also hypothesised that the term Budh listed in an unnamed 9th century Irish manuscript in Wurzburg, defined as both "a point of fire" and the planet Mercury, is a cognate of Budha, the Sanskrit name for Mercury. Budh is also defined in Cormac's glossary as being "Aine's fire".
After Christianity Early Middle Ages The introduction of Christianity into Ireland also resulted in the introduction of Hellenistic astrology to the island. During this period Ireland was known as a centre for the study of astrology and astronomy. Colleges run by the fili, or court poets were the main centres of study; however, monasteries were also centres of astrological activity. Irish chronicles written between 442 and 1133 show a high amount of accuracy regarding astronomical compared to continental sources from the same period. The appearance of the Crab Nebula in 1054 is listed on the same date as that which appear in contemporary Chinese and Japanese chronicles, while Italian sources list the supernova appearing 8 weeks earlier to coincide with the death of Pope Leo I. Aibhistín wrote extensively on the physical and supernatural properties of the Moon, being the first European writer to argue for an influence of the Moon on the tides, and as well as being the first to suggest that the Three Magi who celebrated the Nativity of Jesus were astrologers. An astronomical table spanning 84 years was cited by St. Columbanus during debates with Pope Gregory the Great over the Easter Controversy. Astrological charts in Ireland are recorded as going back to the 8th century, and in 814, the geographer Diciul wrote De astronomiam, a text covering both astronomy and astrology.
High and Late Middle Ages Starting from the 11th century, Arabic astrology brought to Europe from universities led to a standardization of Irish astrology with Continental equivalents. Although Ireland between the 12th and 15th centuries was "a rich source of manuscript texts and books about astronomy and astrology", any trace of pre-Christian astrological systems had been erased at this point.
The Quadrivium and Computus Irish monastic schools taught both the trivium and the quadrivium, with the latter including astronomy and astrology. During the early medieval period, the distinction between astronomy and astrology was not always clearly defined, and astronomical study could incorporate astrological concepts. Joyce describes the "Seven Grades or Orders of Wisdom" associated with the Irish monastic and ecclesiastical schools. The fifth grade, the Foirceadlaidhe, included instruction in enumeration or arithmetic and in the movements of the sun and moon, corresponding to astronomy. Individual monastic centres possessed their own collections of texts and intellectual traditions. Duncan describes the library at Wearmouth, near Bede's birthplace, which was expanded by Benedict Biscop during the late seventh century. Benedict's journeys to Rome brought a large collection of books to England, including calendars, astronomical calculations, and martyrologies containing saints' days and other religious dates. These collections probably included material associated with Dionysius Exiguus's calculations. The precise range of astronomical texts available in Irish monasteries remains the subject of scholarly debate. Nevertheless, Irish monastic centres produced several notable astronomers during the early medieval period. Ellis identifies Mo-Sinu maccu Min, who died around AD 610 and was abbot of Bangor in County Down, as an early Irish authority on astronomy. His pupil, Mo Chuaroc macc Neth Semon of Munster, was reportedly the author of a substantial work on astronomical calculations, although the work is now lost. The writings of Cummian of Clonfert, who died in AD 633, have survived, as has a mid-seventh-century work attributed to Aibhistin, or the Irish Augustine. Aibhistin is considered an early medieval source discussing the relationship between the tides and the moon and may have been influenced by the earlier writings of Posidonius. Irish astronomical learning subsequently influenced scholarship elsewhere in Europe. Irish scholars associated with astronomy included Virgil, who became Bishop of Salzburg in the eighth century, Dungal, who discussed eclipses in the court of Charlemagne, and Dicuil, who was active during the early ninth century. The Irish interest in astronomical timekeeping is also reflected in the ninth-century Sanas Cormaic, which associates the ability to determine the hour during the night with observation of the moon and stars.
The tenth-century Saltair na Rann, traditionally attributed to Oengus the Culdee, similarly emphasizes knowledge of the solar month, lunar age, tides, days of the week, and saints' festivals. These sources demonstrate the integration of astronomical observation with practical timekeeping and the Christian liturgical calendar. The calculation of Easter, known as computus, was particularly important to Irish monastic scholarship. Computus required the reconciliation of solar and lunar cycles to establish the date of Easter and became the subject of prolonged theological and mathematical debate throughout western Christianity. In Ireland, the study of computus encouraged the development of mathematical and astronomical knowledge and contributed to the continued study of celestial cycles. Archaeologist Vance Tiede has proposed that astronomical considerations may also have influenced the architecture of early Irish Christian sites. Tiede examined monastic stone chapels and oratories in Ireland, the Hebrides, Orkney, and Shetland Islands and argued that the orientation of their eastern windows may have been related to the rising sun on the feast days of particular saints. His findings were presented at a 2002 conference of the Historical Astronomy Division of the American Astronomical Society.
Celestial Observations In his article Early Irish Astrology: An Historical Argument, Peter Berresford Ellis cites Hippolytus, who drew on an earlier source in describing the Celts as practicing divination through stars and numbers, in a manner comparable to the Pythagoreans. The question of whether Celtic traditions influenced Pythagoras, or whether the influence operated in the opposite direction, was debated within the Alexandrian School from at least the second century BC and remained a subject of discussion for centuries. McCluskey also discusses Alexander Polyhistor (c. 50 BC), who, in his lost work De symbolis Pythagoricis, sought to locate the origins of Greek philosophy among non-Greek peoples. Polyhistor attributed pre-Greek philosophical traditions to Egyptian prophets, Chaldeans of Assyria, the Druids of Gaul, shamans of Bactria, the philosophati of the Celts, and the Magi of Persia.
Evidence for the continued practice of celestial observation can also be found in early Irish Christian literature. One account concerning a house constructed by the Christian architect Gobhan Saer describes the building as auspicious in relation to the positions of the stars, sun, and moon. This has been interpreted as evidence that some early Irish Christians considered celestial conditions when determining the timing or circumstances of construction. The association of favorable celestial conditions with the foundation of a building may also indicate that such practices retained cultural significance within early Christian Ireland.
Another example appears in a collection of three Middle Irish homilies concerning the lives of St. Patrick, St. Brigit, and St. Columcille. According to the account, the young Columcille was fostered by the priest Cruithnechan MacCellachan. When the boy was ready to begin learning to read, the priest consulted a spaeman, a Scottish term for a diviner, regarding the appropriate time to begin his education. After examining the sky, the diviner advised that the boy should begin learning his alphabet immediately. Columcille subsequently became one of the most prominent scholars associated with early Ireland.
Another narrative provides an example of astrology being used to determine the timing of significant events. Eoghan of Munster encountered a Druid who examined his horoscope and predicted that he would be killed in his next battle. The Druid also predicted that a son conceived at that time would become a powerful king. He therefore arranged for his daughter Moncha to conceive a child with Eoghan. Eoghan was subsequently killed in battle, in accordance with the prediction.
The story continues with Moncha attempting to delay the child's birth until the planetary configuration specified by the horoscope had occurred. She positioned herself against a rock in a stream in an attempt to prevent the birth from taking place prematurely. When the child was eventually born, his head had been flattened by the pressure against the stone. He was consequently given the name Fiachu Muilleathan, meaning "Flathead." The narrative presents astrology as capable not only of predicting events but also of determining the appropriate timing of human actions.
On the eve of Samain, traditionally associated with 1 November and one of the major Celtic cross-quarter festivals, Dathi, King of Ireland (r. AD 405–428), was said to have been staying at Cnoc-nan-druad, or the Druid's Hill, near Skreen in County Sligo. The site was associated with a royal residence. Dathi instructed his Druid to forecast events that would occur during the following year, from one Samain to the next. The Druid spent the night on the summit of the hill observing the sky before returning to the king at sunrise.
Nick Campion notes that interpreting the appearance of clouds was among the methods associated with Druidic divination. The Celtic term naladoracht was associated with this form of divination and also appears to have had an astrological meaning. Campion compares this practice with the Babylonian Enuma Anu Enlil, an eighth-century BC collection of omens that considered atmospheric and celestial phenomena together. Celtic diviners may therefore have employed both stellar and meteorological observations depending on prevailing conditions.