- This family was established at Dromskehy, parish of Drumtarriff, barony of Duhallow, county Cork in the 18th century but by the beginning of the 19th century was settled at Caherduggan, barony of Barrymore. In 1781 Cornelius O'Callaghan married Mary Davies and they had nine children. Two of their sons married daughters of Arthur Pyne of Ballyvolane. Another son was the Reverend Robert O'Callaghan. "The Landed Gentry of Great Britain and Ireland" 1863 and "The Landed Gentry of Ireland" 1958 differ slightly with regard to the descendants of this couple. At the time of Griffith's Valuation members of this family held land in the parishes of Drumtarriff and Kilmeen, barony of Duhallow, Kilnaglory, barony of East Muskerry, Templebodan, barony of Barrymore and Templeroan, barony of Fermoy. In the 1870s Cornelius O'Callaghan of Caherduggan owned 253 acres, Cornelius O'Callaghan of Brookville owned 117 acres, Denis O'Callaghan of Queenstown owned 984 acres and O'Callaghan family members in the Kanturk locality owned over a thousand acres in county Cork. The estate of Leslie Craggs O'Callaghan in the county and city of Cork was advertised for sale in June 1858. In May 1868 an estate of over 1,600 acres in the baronies of Duhallow, East Muskerry and West Muskerry, belonging to Denis O'Callaghan, deceased and continued in the name of his eldest son and executor, Cornelius O'Callaghan, was advertised for sale. The purchasers included Mrs. Lunham and Mr. F. Wyse. Dromahoe and Dromskehy, barony of Duhallow, were advertised for sale in February 1877 and were part of the marriage settlement of Denis O'Callaghan and Sarah Pyne in 1831. In July 1882 the lands of Clonbannin, 105 acres in the barony of Duhallow, the estate of Denis O'Callaghan, were advertised for sale.
- The O'Callaghans of Clonmeen Castle and later of Clonmeen Lodge, barony of Duhallow, county Cork, were established in the Banteer locality until the death of Robert O'Callaghan in 1778. They appear to have held their land from Viscount Lismore. Robert O'Callaghan's sister Melian had married Thomas Morgell in the early 1730s and documents referring to both the O'Callaghan and Morgell families are to be found in the Grehan collection, Boole Library, University College Cork. In 1748 a daughter of Robert O'Callaghan of Clonmeen married Denis O'Callaghan of Glynn.