Trudles,
Backaderry is in the RC parish of Upper Drumgooland, (near Castlewellan). I think that the parish church would be the Church of Immaculate Conception at Leitrim. You could contact the PP to see what birth and marriage records exist for the period you are interested in. You can also check on the PRONI website, as they may have copies, but a personal visit is required to inspect them, and PRONI is closed until April 2011 pending a move of premises.
Elwyn