My only clues for the church your Fairfield might have attended can be found at Public Record Office of Northern Ireland [PRONI] ie
www.proni.gov.uk/ . Their database for 1912 citizens signing the covenant etc contains just under half a million original signatures and addresses of the men who, on 28 September 1912, signed the Ulster Covenant, and of the women who signed the parallel Declaration. In total, the Covenant was signed by 237,368 men, and the Declaration by 234,046 women. I selected three entries because they are in the Knock {Gilnahirk area}. Where they signed ie Bloomfield Avenue Congregational Church might be just the use of a church hall or it could be their church
Fairfield Bella Hazebank Cottage Knock Belfast: East Pottinger Bloomfield Avenue Congregational Church MacRea, E F
Fairfield Jane Cherryvalley Cottages Knock Belfast: South Saint Anne's City Hall Collins, Jane
Fairfield William Hazelbank Colleges Knock Belfast: East Pottinger Bloomfield Avenue Church McKee, John :
I havent a clue if this church exists today, a google search produced Bloomfield Presbyterian Church Cyprus Avenue Belfast BT5 5NT Bloomfield Methodist Church Grand Parade Belfast BT5 5PE Bloomfield Baptist Church 25 Upper Newtownards Road Belfast BT4 3HT Parish Church Bloomfield Road Belfast BT5 5LT Presbyterian Church Gilnahirk Road Belfast BT5 7QQ
Good luck in the quest, Jim