THE KING'S SON 

The descendants of King Brian Boru 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 O'Brien Clan are ranked as one of the chief dynastic families of Ireland.

https://www.familytreedna.com/groups/obrien/

The Y-DNA of the O’Brien Clan is:

L21 > DF13 > ZZ10 > Z253 > BY25450 > FGC5618 > L226> FGC5629

http://ytree.net/DisplayTree.php?blockID=15

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