Quinlan is not a common name in Co Cavan, and one possibility may be that they originated in another part of Ireland. (In the 1901 census, there is only 1 Quinlan household in the county and the husband & wife in it were from Counties Kilkenny & Armagh, so not native to the county at all. They were Church of Ireland).
Do you have any documents recording where Thomas & Ellen were born in Ireland?
What denomination was your Quinlan family?
What was Thomas’s occupation on his 1877 death certificate?
Was Ellen still alive then? (He should be either a widower or a married man, and depending which, you can tell which side of 1877 Ellen died).
If Church of Ireland and born in Knockbride, their records start in 1825. If Presbyterian, their records start in 1764. If RC, theirs start in 1835.