Brian Boru (c. 941 – 23 April 1014) was an Irish king who ended the domination of the High Kingship of Ireland by the Uí Néill. Building on the achievements of his father, Cennétig mac Lorcain, and especially his elder brother, Mathgamain, Brian first made himself King of Munster, then subjugated Leinster, eventually becoming High King of Ireland. He was the founder of the O’Brien dynasty.
In 1000AD, Ireland had a population of less than 500,000 people, with over 150 kings, with greater or lesser domains. My view is that this is indicative of an ancient kingdom, wherein the sons of the sons of the initial king, subdivided the realm of Ireland, over an over again – for a long period of time, going back many millenium.
The descendants of Brian were known as the Uí Briain (O’Brien) clan, hence the surnames Ó Briain, O’Brien, O’Brian etc. O was originally Ó which in turn came from Ua, which means ‘grandson or descendant’ (of a named person). The prefix is often anglicized to O’, using an apostrophe instead of the Irish síneadh fada (´). The O’Brien’s subsequently ranked as one of the chief dynastic families of the country (see Chiefs of the Name).
The O’Brien Clan are a member of the Irish Type III Cluster within L226 SNP:
http://www.irishtype3dna.org/index.php#howold
https://www.familytreedna.com/groups/r-l226-project/about
Details on other key ancient Irish dynasties, including Clan Colla and Niall of the Nine Hostages (Ui Neill) can be found at:
http://www.peterspioneers.com/homepage.htm
The geographically disperse locations of the Celtic Nations can be found at:
https://en.wikipedia.org/wiki/Celtic_nations
Also information on the Royal Alpin House.
The Y-DNA of the O’Brien Clan is:
L21 > DF13 > ZZ10 > Z253 > BY25450 > FGC5618 > L226 > FGC5629
https://www.familytreedna.com/groups/obrien/about/background