Okay, sure, since you asked –
First of all, it’s important to distinguish between clan names and family names. When you say, “The McGonnicals were one of the 50 families,” it’s probably more accurate to say Ireland was made up of 50 small semi-nomadic kingdoms. People with the last name Connelly today are more likely to be descended from a subject of a king called Maachoneri than King Gonerill himself. Besides, “Conroy” is actually an Anglicized version of the original Irish clan name, and multiple Irish families had their names Anglicized as Conroy. Joyce has a similar problem. Yes, there may have been a battle where a Joyce beat a Conroy, but we don’t know if they were our Joyces because there were Joyces in the other British Isles who may have arrived after that important battle, and maybe these are our Joyces. Who can know?