The most famous Irish traitor, William Joyce, aka "Lord Haw Haw."
Who exactly was he a traitor to?
He was born in the U.S., informed on the IRA to the British while living in Ireland where he grew up, and then made propaganda broadcasts for the Nazis (he had earlier become a member of the British Union of Fascists). He was also a German citizen. Seems like he betrayed everyone (other than his last chosen nation).