John:
Normally, a marriage occurred in the bride's parish. The marriage of Michael and Brigid occurred in Castlerea not Fairymount which would imply that Brigid was from the Castlerea area but maybe the marriage was held in Michael's parish. Below are links to the Griffiths Valuation head of households for Tibohine and Kilkeevin civil parishes. Tibohine which includes Fairymount has a great number of Lavin records. Kilkeevin civil parish which includes Castlerea has a smaller number of Lavins. You will note there is a Michael Tiernan in Ballindrumlea.
I looked at the Fairymount baptismal records which start in 1833 for Mary Lavin records and there were two in 1833, one in 1834 and two in 1836. Also Mary could have been older and not shown on the baptismal records.
Roger McDonnell
https://www.failteromhat.com/griffiths/roscommon/kilkeevin.htm
https://www.failteromhat.com/griffiths/roscommon/tibohine.htm