apparently Gabhaim Molta Bhríde is not all that old as traditional songs go, although the lyrics are written in an old style
The song is attributed to Tomás Ó Flannghaile (1846-1916. Ó Flannghaile was born in Co Mayo, and was a native speaker of Irish, but he was only about 6 years old when his family moved to Manchester. Ó Flannghaile later taught at a training college in London. Among his other songs was "Dóchas Linn Naomh Pádraig", a hymn to St Patrick.


http://homepage.eircom.net/~oflannery/bio/bioTOF18461916.htm