As you probably know, Drumgoon/Cootehill RC parish records go back only to 1829, so unless Philip and Elizabeth married in 1829 or early 1830, there's not likely to be a record of their marriage. The Griffith's index shows Philip McMahons in Cavan, but none in Drumgoon. You might check the Tithe Applotments, done in 1820s and 1830s. Diane