As you probably know by now, civil registration of Catholic marriages only began in 1864 so if they were Catholic and married before this date you will need to find a church record, if it still exists, of the marriage. The wedding most likely took place in the bride's church which may or may not be the one she attended after her marriage. Have you searched the marriage indexes yet?
Three children to James Gartlan & Judith Slevin listed on IGI:
1. Sarah born 12 Sept.1868 Co.Monaghan.
2. Patrick born 6 Mar.1871 Co.Monaghan.
3. Ane born 5 July 1873 Co.Monaghan.