Statutory birth registration started in Ireland in 1864. I searched the birth records for the area around Kilmore, for children born to a couple named Finnegan and Curtis. I didn't find any, so that suggests that they had stopped having children by that year, or had died or moved away.
Do you know anything about Peter’s occupation? (From a child’s death or marriage certificate perhaps)? Was he a farmer? If he was a farmer, then you would expect him to be in the tithes c 1833 and Griffiths Valuation in 1864. However if he was a labourer or weaver, then he may not be in those records and would be harder to trace.
I searched the tithes for a Peter Finnigan, and found just one. He lived in Drumnahunshin (spelling varies), and a George Finnegan was also farming there:
So if your ancestor was a farmer, then this might be him, but if he was a weaver/labourer or some other occupation it might not.
By 1901, George is still farming in that townland:
Peter has disappeared. There are a couple of possible deaths in 1875, aged 95 & 1876 aged 75. You would need to pay to view them to see if either lived in Drumnahunshin. Both would be a bit old for a marriage in 1846, but it might have been a second marriage for Peter, you never know.
I looked for possible deaths for Isabella 1864 – 1901 but didn’t see any.
The surname Curtis only appears once in the tithes and that’s for a Felix who lived in Kilmore townland. Again, do you have any knowledge of Isabella’s father’s name and occupation?