I see a Catholic baptism record (and scan) for the 1845 marriage, which I presume you have
I also saw the baptisms of Patrick in 1846 and Edward in 1863, in Ballynahatten, both in Kilkeel
In the 1864 Griffith valuation, Nicholas is living in Ballynahatten, on 2.6 Irish acreas and a house woth 10s rent pa, a relatively modest cottage, I suggest. This is the property he leased, a most peculiar oval shape (in the left hand image, toward lower left), right in the middle of a river, from the look of it. There is no house marked, but these maps weren't drawn up at the same time as the Griffith valuation, so he may have built a house himself.
https://1drv.ms/u/s!AjooUIUtx9j5ha4XeaxZ65eXK03HoQThe property records show he had died by 1874, but I don't see a death record for him.