Looking for Mary McGovern

I'm looking for information on Mary Alice McGovern, born 1857 in Kill townland, Crosserlough civil parish, Cavan County. Her daughter was Mary Alice McDonald, my grandmother. On my grandmother's birth certificate it states Mary Alice McGovern and John McDonald were married in "Kill, Cavan" on 1 June 1878. However, even with that information I have found no record of the marriage, though I've contacted churches in Cavan and investigated through Legacy and PRONI. No records have been found.

I'm assuming Mary McGovern was Catholic, as my grandmother was raised Catholic (and later excommunicated for marrying my grandfather, who was Presbyterian, even though it was largely an arranged marriage). I don't know whether John McDonald was born in Ireland or in Scotland, and have found nothing on him as yet.

If anyone can suggest any leads, I'd be most appreciative.

Judy