The ancestors of the Ua Ruairc clan were originally among the lords and kings of a group known as the Uí Briúin Bréifne. Bréifne was a small kingdom which is roughly equivalent to the modern counties of Leitrim and Cavan (see map of Ireland). O'Rourke country was later described as Breffny O'Rourke (county Leitrim), and they were chief lords in this area until the 1600's. For further reference on Breifne, see this article on Ua Ruairc of Bréifne.
According to the Irish genealogical tradition, the O'Rourke pedigree traces its ancestry back to
Conn Ceadchadhach (Conn of the hundred battles), whose progeny included many of the so-called High Kings (Ard Ri) of Ireland. From Conn the legendary lineage of the High Kings continued with Conn's son Art Aonfhir (the Lonely) to his grandson Cormac MacAirt. Cormac was the father of Cairbre Liffechar (of the Liffey), who was the father of Fiachadh Sreabhthuine, who was the father of Muiredeach Tireach, who was the father of Eochach Muighmedon. From Eochcach's son, Brión, a King of Connacht in the 4th century, descends the royal line of the Ui Briuin (the race of Brion), whose descendants were kings in the province Connacht (western Ireland) for many centuries. Brión is recorded with 24 sons, one of those being named Duach Galach. Duach is described as a common ancestor of the O'Conors, the O'Flahertys, the O'Rourkes, the O'Reillys, and other noble familes of Connacht.
According to the Irish genealogies of O'Clery, et. al., Duach (Dauí) Galach had a son named Eoghan Sreib. Eoghan's son Muiredach Mal had a son (or brother) named Fergus. From Feargna, the son of the latter Fergus, descended the tribal group of the Uí Briúin Bréifne (the O'Rourkes and O'Reillys among others). Sometime around the 6th century Feargna is said to have migrated to the Bréifne area, although this is highly disputed. Feargna was succeeded by a son named Aedh Finn (Hugh the fair) who died about the 7th century. From Aedh's son, Scanlan, was said to follow in order of descendancy: Crimhthann, Fedlim, Blaithmac, Baithin, Donchadh, Dubhdothra, Cernach, Sellachan, Tigernán, and finally to Ruarc, the namesake of the O'Rourke's (et al).
Cautionary note: Francis J. Byrne in his Irish Kings and High-Kings (2nd edition, 2001) states the early "Uí Briúin pedigrees show every sign of falsification. As the dynasty was comparitively insignificant as late as the 7th century, and spread out from central Connacht in the course of the eighth, we should look for a parallel expansion of its genealogical ramifications at approximately the same date." He goes on to state the various Uí Briúin "each trace their separate descent through lines of unrecorded or dubious ancestors to Brión and his suppositious son Dauí in the fifth century."
Modern Irish historians agree it was not unusual for the earliest genealogies to have been crafted for political reasons, and as a means of connecting the more important septs of the day to one another.
It is considering this perspective the following Ó Ruairc pedigree is reproduced from the older Irish genealogies, with perhaps Tigernán of the 9th century being the most tangible of the early Ó Ruairc ancestors.
An O'Ruairc Pedigree
(1) Eochu Mugmedón, died after 360 AD, son of Muiredeach Tireach
(2) Brión - ancestor of the Ui Briuin of Connacht
(3) Duach Galach
(4) Éogan Sreibh
(5) Muiredach Mál *
(8) Aedh Find
(11) Fedlim *
(12) Blaithmac *
(15) Dubh Dothra (obit. 743)
(16) Cernach (or Cernachan)
(18) Tigernán (obit. betw. Bet. 888 - 892)
(19) Ruarc mac Tighernáin (from whom the surname O'Rourke derives)
(20) Art * mac Ruairc
(21) O Ruairc, Sean Ferghal, King of Connacht: 956-964/7 (a chart of the Ó Ruairc kings)
(22) O Ruairc, Aedh - died abt 1015
(23) O Ruairc, Art Oirdnide (Uallach), King of Connacht: c.1030-1046
(24) O Ruairc, Niall - died abt 1047
(25) O Ruairc, Ualgharg - died abt 1085
* Notes on the names appearing in green: The earliest genealogies, those from the Book of Leinster and from Rawlinson B502, do not indicate the individuals whose names appear in green. Instead, they first appear about the time the Book of Ballymote was transcribed in the late 14th century. It is possible, given the information from these earlier genealogies, that Muiredach Mál was a brother of Fergus, not his father. The name Muiredach Mál may have been inserted to more closely match the O'Ruairc pedigree back to that of O'Conor of Connacht.