The spelling of Kirwan seems to be purely Irish, and found in County Galway -- where the Kirwans originated before the Romans showed up. The Kerwin spelling is anglicized and is found in the Dublin area during the 1600's and up until today. My Kirwan ancestors spelled their name Kerwin and Kirwan, which is puzzling. Kerwin was also a rather common spelling with census takers. Since a great many of our Irish ancestors were illiterate, the spelling of their last name was up for grabs. Carole Shanahan