David and Cathy:
Welcome to Ireland Reaching Out!
All of the surnames you mention are common in Co. Donegal.
Finding RC records prior to 1850 is very difficult in Co. Donegal. Only a few parishes have records pre-1850. Kilmacrenan RC records start in 1862 and Letterkenny records start in 1853. Civil registration did not begin until 1864. This explains why you have been unsuccessful in the last twenty years trying to find records for your ancestors.
There is no official townland called Glenvar in Donegal so possibily it was a local placename. There are two townlands called Glencar Irish and Glencar Scotch near Letterkenny in the civil parish of Conwal.
To recap, Kilmacrenan is a small village in Donegal within the fairly large civil parish of Kilmacrenan and there is also an RC parish of Kilmacrenan. This parish is just north of the civil parish of Conwal with the very large town of Letterkenny and the townlands I mentioned (Glencar Irish and Glencar Scotch) are within this civl parish. The RC parish is Letterkenny.
Let me know what questions you have.