: any information going back plus dates.
I can tell you that my Finns lived in the Townland of Lisduff (Fairymount area).
Directly across the road from the old Finn homestead lived a family of Regans.
This family of Regans owned and operated a large tractor company in the nearby
town of Frenchpark. One of the Regan girls (Philomena Regan) came to the U.S and
married a Brennan.
Philomena is recently widowed and lives in Middletown, New Jersey.
A search of the Griffiths Valuation on this site (which is complete for your area
of interest) in the civil parish of Tibohine, reveals 2 Cullinanes and 2 Cullinans.
The Griffiths Valuation data on this site represents the time period between 1857-1858.
Unfortunately, a search of the Elphin Census(1749) for all of Roscommon reveals only
1 Cullinan in the townland of Charlestown, parish of Killmore.
I centered my searching on your cullinan surname however, needless to say, you
can do the same search on Regan and additionally in the 1901 Census.
Good luck with your research.