Ancestry has created a searchable surname index to another record collection held by the Public Record Office of Northern Ireland. It's the Armagh and Londonderry Absent Voters Lists, 1918.
In that final year of WW1, special provision was made for people serving in the military, the Merchant Marine and the Red Cross to vote while away from home, and these lists survive for just two Irish counties.
The index for Armagh holds c.3,500 names; the list for LondonDerry c.4,500.
The Ancestry collection links to the relevant page of the original lists, while on PRONI a pdf of the lists for each constituency is freely available to download for Armagh and LondonDerry.