By the time of Griffith Valuation 1860 Fivemiletown was in the Clogher Union, Parish Clogher, Tyrone. Parishes could expand, contract, join and leave other Unions depending on the Diocese.
There were both Ross and Thompson but none in Fivemiletown but this is just 1860. The Tithe of the 1820's could give you a different snap shot in time.
Fermanagh is Northern Ireland and is the telephone directory. There are just 2 surnames Ross, one Enniskillen and one Derrhillagh. 30 Thompson's, but not specifically Fivemiletown a good map could help you.
Tyrone in the NL directory and there you can breakdown to Fivemiletown, Tyrone and there are 12 Thompson's and 2 named Ross. In Northern Ireland the postal codes are included with the phone numbers.
While waiting for contacts you can write some letters.
It is unlikely the families set sail from Liverpool, but ship might of as the ships then stopped at Irish Ports on their way to North America.