For Rob(ert) Farrell M 1850 +/- 2 yrs, Co. Leitrim, I found 1 rcd, 1850, Kiltoghert RC parish (your info was good!).
For Cat(herine)(Kate) McDermott M ditto, I found ditto. These are HIGHLY likely to be for your ancestors. A view of the detailed rcds SHOULD reveal their parents' names. This is useful in identifying and validating earlier generation's B/M rcds.
For the children assuming they were all born in Kiltoghert RC parish, I found a John, 2-Thomases, and a James, but NO Mary Elizabeth. There is a "Bessy" B 1852.
For Rob(ert) Farrell B 1825 +/- 5 yrs, in Cos. Leitrim, Longford, Cavan, Sligo, and Roscommon (these surround Co. Leitrim), I found 5 potential rcds: 1826 Longford, 1823 Roscommon, 1822 Longford, 1828 Longford, and 1821 Roscommon.