There is a marriage of a Patrick Dunn
and a Mary Brien also a Mary Bryan (possible the same person with two different spellings)
in 1880 registered in Ballinasloe
all three have the same vol and page number but the images of these records are not online yet
http://www.rootschat.com/links/01jv2/If this is the correct marriage there may be more children before Mary Ann.
as the image is not online it may be possible to find it in the church records.