Just to add fuel to the discussion. Perceived wisdom over this side of the "puddle", (dependent on where you read it!) is that the Gleesons form the main part of the Clan and are based around the Tipperary/Limerick region in the Southwest of Ireland. The Gleasons are a small sub-clan clustered around the town of Limerick. Both they and the English Gleasons are said to be decended from the Glissaines, knights who came over from Normandy with William The Conquerer in 1066. Maybe they were then given estates by him as reward for services rendered and the names susequently corrupted by the Saxon or Gaelic ancestors of us all. I have almost given up trying to trace my ancestors over there from over here. Good luck to those of you with the tenacity to carry on.