I searched the Irish Family History Foundation's online database. Rcds B4 1840 are SPARSE. All surviving rcds for Cavan are in the database.
For a marriage of John Lynch (+variants) and Anne Brady (+variants), I find paired rcds for each in the parish of Lurgan in 1838. If you view a detailed rcd, it SHOULD show you the names of their parents, and sometimes other info such as ages, residence, etc. These can be clues to moving back another generation.
For a possible birth date for Anne Brady, there are 3 rcds in Lurgan parish in 1822. You are likely to find siblings for her in this parish.
For a possible birth date for John Lynch, there are 6 rcds in Cavan. None are in Lurgan.