North Tipp Parishes; McGraths to Boston in 1848, common given names

Hello All,
A garda at Shannon Airport customs recently told me I couldn't throw a rock in the parking lot without hitting another McGrath in the head. By the looks of the records, he may have been being humorous, but I'm thinking he wasn't really joking.

In trying to make the jump back to Ireland from Boston, I'm running into some difficulty narrowing down the possible parishes; a common surname, and common given names too -

John and his older sister Eliza left the port of Galway on the Brig Demerara in 1848 and landed in Boston, MA; he was 17, she was 21 yrs old. Their father's name was William McGrath. No luck thus far on a mother's name, or other siblings, and nothing definitive on a parish. John married Hanora Murphy in Boston in 1853 at 23 yrs old, had a family which included my g-grandfather, John William. I've found nothing on Eliza. 

I'm starting in on North Tipp and Clare, thinking this; that if John and Eliza were walking to a port, it's more likely they would go to Cork or Cobh if they were starting in southern parishes, though the McGraths seem to be numorous there. Griffith's gives me a short list of parishes with a William in Tipperary North Riding, including Castletownarra, Youghalarra, Bergesbeg - northern/close to Clare, much shorter walk to Galway. Ballycahill, too, though it's getting farther from port. 

I'd be happy for any suggestions that anyone may have on where I might go next. (even a couple of professionals have very politely mentioned it is going to be difficult if possible at all...)

thanks for reading. 

Pete