One border of Lough is shared with Armagh.
You don't give any dates? So here are some resources
The Northern Ireland telephone book, for Down an Armagh
Ireland telephone book for Louth. Look for Dundalk addresses
in the phone book and the same in Northern Ireland.
If they were in the UK by 1871 I have a census that identifies Irish born persons.
Griffith Valuation for all counties, different years from 1847-1864.
1901/1911 Census: www.census.nationalarchives.iewww.genealogy.nationalarchives.ie
has records for England and Ireland after 1864 up to the 1950's.
Protestant marriages civil registration from 1845.
Scarva, Down could be the village or the townland, in the Parish of Aghaderg and the Poor Law Union of Banbridge.