After contacting a significant number of Gilmartins in Ireland in a non-scientific poll, it would appear that most originally came from co Tyrone (many 100's of years ago) but in the last 200 years, most came came from co Sligo, co Leitrim, or co Mayo. There are also the odd Gilmartins (e.g. from co Tipperary, co Kilkenny, etc.) who apparently migrated on past co Sligo and co Leitrim. Some of the Gilmartins from my informal (and anecdotal) survey have volunteered to take the yDNA test to start the process of matching up yDNA with Gilmartins in America.