According to the Milesian Genealogies, the O'Domhnallains are descended from the line of Heremon - one of three sons of King Milesius who settled in Ireland. It was not until the 11th century that the great Irish King, Brian Boru, decreed that all inhabitants of Ireland should adopt surnames. These surnames were selected from great warriors, poets, kings, and so forth in Irish history.
My study of the Chronicles indicate that there were 5 identifiable Donnellan septs existing about 1500 AD:
1. Donnellan #1 of Connaught
2. Donnellan # 2 of Massarene
3. Donelan of Ballydonelan
4. O'Donnellan of Hy Tuirtre
5. O'Donellan, Chiefs of Clan Breasail
I can find verifiable familial relationships between Septs
1, 3 and 5 referenced above
Septs 2 and 4 appear to have little surviving record of their existence after 1500 AD, hence leading me to believe they have disappeared during and after that time. Small wonder with the combined circumstances of Civil Wars; Wars of Independence; Cromwell; Williamite War; Penal Laws; Famine(s) and more.
In the Annals of the Four Masters, we see recorded the O'Domhanallain name for the first time. These Annals, first translated by four Monks in the 14th century, are some of the earliest guides to the historic Donnellan/Variants family(ies). For a sampling of Donnellan references in these Annals, click on ANNALS.
The Tribes and Customs of the Hy Many is John O'Donovan's translation of the Book of Lecan which was transcribed for Gilla Iosa More Mac Firbis, chief historian for the O'Dowds about the year 1418. For our purposes, it is most notable for positioning of the O'Domhnallain sept in the Roscommon and Galway sections of O'Kelly's country in medieval and earlier Ireland.
O'Donovan's book contains a number of passages relating to the O'Domhnallains. In Note F to the book, there is a 5-page pedigree on the O'Donnellan family that traces the family along the Milesian lines - through Archbishop Nehemiah's lineage - and along his eldest son John's line to the 19th century. I initially used this pedigree to both verify its integrity and extend to other sons (Edmund, James, William, Teige and Murtough). The search has been both challenging and edifying and I hope will be of interest to others who are interested in the Donnellan family history. Perhaps there is even a bloodline connection in there for you.