Since he is your great uncle it sounds like you might know who his parents and at least one sibling are, and in that case I would think the first order of business is to locate his family in the 1911 census.
From that you will have a place, and with any luck he might have been living in the same place when he got married. That might help you narrow down marriage records.
I just looked at the 1911 census and do not see McCarthy or MacCarthy in Kilfaughabeg DED, where Kilbeg would be located.