Barb,
Meant to add, I do notice in the 1911 census there are McCarthys in Baltimore and a townland called Letter in a D.E.D. called Crookhaven, which I don't think is the right Letter.
I'm heading out right now to the family history center to look at McCarthys in 1901 census film - if I happen to stumble across Baltimore, Lettertinish, and Lough Hyne, I'll try and copy it.