I just did a quick search of the Eircom phonebook, and indeed it looks like there aren't McCarthys in Baltimore, Letter (or Lettertinish), or Lough Hyne.
I've been told my McCarthys are McCarthy Sowney (Samhna). From googling around (and if the information is correct) there were two main branches of McCarthys, the Mor branch and the Reagh branch.
Within the Reagh branch are Samhna (Sowney), Foirsin, and Carthaigh. (This information is NOT easy to come by, so I cannot vouch for the accuracy!)
So there could be a connection, but it likely precedes the 19th century. I don't think my McCarthys lived down toward Baltimore. I think mine were more from Lissane, in Caheragh parish, Drimoleague and particularly Dunmanway areas.
I've been in touch with another McCarthy hunter (Leila Mayberry) and she and I seem to have family trees tangled up with each other. Her Margaret McCarthy looks very suspiciously like my great grandmother, but the two Ellens (sister of Margaret) we have just don't seem to match. So I have sent a research query to the GRO with a list of references to 17 Ellen McCarthys born 1870-1874 in this area, with a request to send me a copy of the birth record of any Ellen whose father is Cornelius and whose mother is named Mary. It should take about six weeks.