Welcome to Ireland Reaching Out!
I also was unable to find the marriage record. I saw their son Patrick's baptismal record in March 1879 in Ballinrobe but searches of the subscription site Roots Ireland (for all of Ireland) plus www.irishgenealogy.ie came up empty. The 1911 census indicates a marriage around 1875-1876. I also scanned the Ballinrobe marriage register from 1875-1880 with no success.
Where is Castlegrove?