Most, but not all, were resident in the six counties that now make up Northern Ireland.
If you're getting a sense of groundhog day, I'm not surprised. It was only six weeks ago that a new index to this collection appeared in Ancestry's database, and the records themselves have been freely searchable online via the Public Record Office of Northern Ireland's website (here) since 2012 – it went 'live' on the centenary of its creation.
It's appearance on both Ancestry and now FindMyPast marks PRONI's 100th anniversary, and the two new indexes link to PRONI's images.
All three databases are free to search by name and address.