I searched the Irish Family History Foundation's database. It contains all the surviving rcds for abt 60% of Irish counties. Rcds for Cavan are abt as good as they will ever be. In all counties, rcds B4 1840 are sparse.
For Felix Shannon in all Ireland, I found 1 birth in 1916, 0 marriages, and 1 death in 1867 Leitrim. Since there is no M rcd for Felix, there would be no M rcd for Ann.
For Patrick, John, and Phillip B 1824 +/- 4 yrs, in Co. Cavan, I found 0 B rcds.
I conclude that rcds for these individuals in this range of yrs were never kept, or have been lost through time.