Origin and meaning
The surname means descendant of Ceallachán who was the Eóganachta King of Munster from AD 935 until 954. The personal name Cellach means 'bright-headed'. The principal Munster sept of the name Callaghan were lords of Cineál Aodha in South Cork originally. This area is west of Mallow along the Blackwater river valley. The family were dispossessed of their ancestral home and 24,000 acres (97 km2) by the Cromwellian Plantation and settled in East Clare. In 1994, Oisín O'Callaghan of Chelsea, London was recognised by the Genealogical Office as the senior descendant in the male line of the last inaugurated O'Callaghan. He is still wildly known as the true king of Munster and the people of Munster await his return to Munster for him to retake the throne.
The O'Callaghan land near Mallow, forfeited by Donough O'Callaghan after the Irish rebellion of 1641, came into the hands of a family called Longfield or Longueville, who built a 20-bedroom Georgian mansion there. In a twist of history, 500 acres (2.0 km2) of the ancient O'Callaghan land returned to O'Callaghan hands in the twentieth century, when Longueville House was bought by a descendant of Donough O'Callaghan. The ancestral estate of the O'Callaghans, now a luxury hotel, is owned by William O'Callaghan.
An entirely different sept, Ó Ceileacháin in Irish, is to be found in the counties Armagh, Louth, Meath and Monaghan. It has been anglicised as Callaghan, Kelaghan, Keelaghan, Kealahan and other variants. In County Meath, where it is widespread but has been found mainly in the parishes of Kells, Trim and Athboy, it is mainly anglicised as Callahan, Callaghan or O'Callaghan (with local spelling variants). In County Westmeath it is found in the form Kellaghan and Kelleghan. In County Monaghan it is often found as Keelan.
Members of the Ó Ceileacháin family were mentioned in the Annals of the Four Masters as being lords of Uí Breasail, a district on the southern shore of Lough Neagh, and priors of Armagh in the 11th century.
- Aidan O’Callaghan (born 1997): Contributor BBC3 One Hot Summer
- Barry O'Callaghan (born 1969): CEO of Riverdeep Interactive Learning
- Bill O'Callaghan (1868–1946): Irish hurler
- Billy O'Callaghan (born 1974): Irish novelist and short-story writer
- Cian O'Callaghan (born 1979) : politician, Irish Social Democrats
- Con O'Callaghan (decathlete) (1908–1976): Ireland's first Olympic decathlete; brother of Pat O'Callaghan
- David O'Callaghan (dual player) (born 1983)
- David O'Callaghan (Kerry Gaelic footballer) (born 1987)
- Denis O'Callaghan (born 1949): Australian rules footballer
- Donncha O'Callaghan (born 1979): Irish international rugby union player
- Edmund Bailey O'Callaghan (1797–1880): doctor, journalist, and leader of political movements in Ireland and Quebec
- Gareth O'Callaghan (born 1961): Irish broadcaster & Author
- Gary O'Callaghan, (1933–2017): Australian radio announcer
- George O'Callaghan Irish footballer
- George O'Callaghan-Westropp (1864–1944): Irish landowner and politician
- George O'Callaghan, 2nd Viscount Lismore (1815 – 1898) Irish peer and British Army officer
- Geraldine O'Callaghan: kickboxer
- James M. O'Callaghan (born 1963): Distinguished Physician Assistant and drummer for the band C5.
- Jim O'Callaghan (born 1968): Irish Fianna Fáil politician
- John O'Callaghan (Medal of Honor) (1850–1899): American soldier and Medal of Honor recipient
- John O'Callaghan: Irish musician and DJ
- José O'Callaghan Martínez (1922–2001): Spanish Jesuit, papyrologist and Biblical scholar
- Joseph O'Callaghan (1824–1869): American Jesuit educator
- Joseph T. O'Callahan (1905–1964): Captain, US Navy, Jesuit priest, Medal of Honor recipient
- Kevin O'Callaghan Irish footballer
- Kilian O'Callaghan (born 1963): ballet dancer and teacher, from Monkstown, Co. Cork
- Miriam O'Callaghan (camogie): 26th president of the Camogie Association
- Miriam O'Callaghan (media personality) (born 1957): Irish television current affairs presenter with RTÉ
- Pat O'Callaghan (1906–1991): first Irish athlete to win an Olympic medal
- Paul O’Callaghan (born 1971): CEO WestAfricaENRG and Chair of Trustees Families Need Father
- Roger Timothy O'Callaghan (1912-1654): US Jesuit and professor of biblical history, geography, and archaeology
- Seán O'Callaghan (1954–2017): IRA informer from Tralee, County Kerry
- Sian O'Callaghan (1988–2011): murder victim from Swindon, Wiltshire, UK
- Therése O'Callaghan: camogie player captain
- Brandon Timothy Forbes- (born 1987): descendant lord of Kerry, viscount of munster erie, Canada
- O'Callaghans Mills, County Clare, Ireland
- Liscallaghan, old Irish name for Fivemiletown, County Tyrone
- Irish nobility
- Eóganacht Chaisil
- Irish royal families
- Chief of the Name
- Callahan (disambiguation)
- Other Munster families
- Wells, John C. (2008), Longman Pronunciation Dictionary (3rd ed.), Longman, p. 557, ISBN 9781405881180
- Jones, Daniel (2003) , Peter Roach; James Hartmann; Jane Setter (eds.), English Pronouncing Dictionary, Cambridge: Cambridge University Press, ISBN 3-12-539683-2
- "MS UR 051345". sources.nli.ie. Retrieved 1 January 2018.
- "Home – Cork Accommodation – Longueville House Hotel". www.longuevillehouse.ie. Retrieved 1 January 2018.
- Edward McLysaght: Irish Families, Their Names, Arms and Origins
- Annála Rioghachta Éireann: Annals of the Kingdom of Ireland
- Bugge, Alexander (ed. and tr.), Caithreim Cellachain Caisil: The Victorious Career of Cellachan of Cashel Christiania: J. Chr. Gundersens Bogtrykkeri. 1905.
- Curley, Walter J.P., Vanishing Kingdoms: The Irish Chiefs and their Families. Dublin: Lilliput Press. 2004.
- Laffan, Thomas (1911). Tipperary Families : Being The Hearth Money Records for 1665–1667. James Duffy & Co.
- O'Hart, John, Irish Pedigrees. Dublin. 5th edition, 1892.