The Griffith's index seems to bear out the family Killeevan memories, as the majority of Quigley entries are for that parish, and there are also Cassidys, Wards, and McQuillans there. I didn't find Michael's death in the civil death index, but there are some possibilities for Mary (Cassidy) Quigley (born before 1801 if son born in 1816): 2 in Clones district (1783-1869 and 1795-1866), 2 in Castleblaney (1786-1879 and 1800-1897), and 1 in Dundalk (1792-1874). Killeevan is in Clones district, so you might get those death records. You couldn't prove she was your Mary, but if a widow in Killeevan, that might be another hint. You could do the same with Bridget (McQuillan) Ward, born before 1808, hopefully died after 1863. Diane