I research the Irish Family History Foundation's database. It contains 12,000,000 surviving rcds for abt 65% of Irish counties. Rcds are increasingly sparse B4 1840, rare B4 1800.
Note 1 - Without a KNOWN county, the chance of an ancestor's rcds NOT being in this database is AT LEAST 35%.
Note 2 - Only 1/2 of Co. Cork's rcds are in this database.
Note 3 - The RC church kept control over most parish rcds from Tipperary (north). These are ONLY available thru the Tipperary Family History Research center at a substantial fee (regardless of outcome).
For Robert Ferrell D 1906 (exact), Co. Leitrim, I found 1 rcd. A view of this detailed rcd MAY reveal info on his parent's names as well as other info. This MAY help you identify his and her B/M rcds.
For Thomas Ferrell B after 1850, in selected counties, with a father forenamed Robert, I found 3 rcds: 1855 Leitrim, 1858 Leitrim, and 1859 Limerick.
For Mary Jane Dandie B 1850s, Co. Cavan, with the father's forename unknown, I found 2 potential rcds: 1855 and 1859, both in Anna CI parish. Father's forenames were both Henry. I couldn't find a M rcd for her at any reasonable date.
For James Ferrell B ditto, I found only 1 rcd within a relevant range: 1851 Leitrim.
For Mary Ferrell B ditto, I found 4 rcds within a relevant range: 1847 Longford, 1854 Cavan, 1858 Galway, and 1860 Sligo.
For Robert Ferrell M 1850 +/- 4 yrs, somewhere in Ireland, I found 4 rcds: 1846 Galway, 1850 Leitrim, 1851 Limerick, and 1854 ANT. The Leitrim rcd should be slightly favored as it is closest to Carrick-on-Shannon.
For Robert Ferrell B 1825 ditto, with the father's forename unknown, I found 14 rcds: 1821 ROS, 1821 DUB, 2-1822 DON, 1823 ROS, 1824 ARM, 1825 WIK, 1825 ARM, 1826 LNG, 1827 WSM, 1828 KID, 1828 LNG, and 1828 KID. Will leave it to you to find the respective father's forenames.
Google the Foundation's name to lead you to this database. Register, opt to be notified as rcds are added and browse. Pls post your search results to my genforum msg.