There is a Thomas Joseph McDermott of Carrick on Shannon, born about 1863, RC, farmer and then hotel proprietor, unmarried and living with his mother (the hotel proprietor before he was) in 1901 and married to a Mary E by 1911. He died in 1916, age 53, and admon was granted to his widow in 1917. His estate was £2,878, resworn as £4,168. If he was the father, maybe they didn't marry because they were from different classes?
Apparently Leitrim County Library has some minute books from Carrick on Shannon Union, which might have info about Bridget and Thomas:
http://www.workhouses.org.uk/CarrickOnShannon/