I just realized I didn't answer one of your questions in the above reply.

Yes, John Skillicorn's mother was Elizabeth Ceiggeen.

It somewhat of a leap to assume that Isabel Kegeen, wife of Gilbert Callin, and Elizabeth Ceiggeen, wife of Gilbert Skillicorn, were sisters. However these couples were tenants of half-quarterlands of Ballaragh, and the easiest explanation of this is that Isabel and Elizabeth were sisters, and had inherited.