Duach mac Dallán was the 6th king of the Irish kingdom of Uí Maine, around the 5th- or 6th-century.
John O'Donovan remarked that "Dallan, who was also a brother of Fiachra Finn, was prince of Hy-Maine for eleven years, when he was mortally wounded and afterwards drowned."
References
http://www.rootsweb.ancestry.com/~irlkik/ihm/uimaine.htm Annals of Ulster at CELT: Corpus of Electronic Texts at University College Cork Annals of Tigernach at CELT: Corpus of Electronic Texts at University College Cork Revised edition of McCarthy's synchronisms at Trinity College Dublin. Irish Kings and High-Kings, Francis John Byrne, Dublin (1971;2003) Four Courts Press, History of the O'Maddens of Hy-Many, Gerard Madden, 2004. . The Life, Legends and Legacy of Saint Kerrill: A Fifth-Century East Galway Evangelist by Joseph Mannion, 2004. http://www.ucc.ie/celt/published/G105007/index.html