O'Callaghan (/əˈkæləhən, oʊ-, -hæn, -ɡən, -ɡæn/), Ó Ceallacháin, or simply Callaghan without the prefix, is an Irish surname. Often when Irish migrated to the United States and the "g" was removed from the spelling in the ships' manifests.
The surname means descendent 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, Don Juan O'Callaghan of Tortosa was recognised by the Genealogical Office as the senior descendant in the male line of the last inaugurated O'Callaghan.
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.
Ernie Callaghan (1910–1972), association footballerTabby Callaghan (born 1981), Irish guitarist and singer-songwriterBarry O'Callaghan (born 1969), CEO of Riverdeep Interactive LearningBill O'Callaghan (1868–1946), Irish hurlerBilly O'Callaghan (born 1974), Irish short story writerCon O'Callaghan (decathlete) (born 1908), Ireland's first Olympic decathlete; brother of Dr. Pat O'CallaghanChristopher Callaghan, 2006 Republican candidate for New York State ComptrollerDaniel J. Callaghan (1890–1942), Rear Admiral, US NavyDavid O'Callaghan (dual player) (born 1983)David O'Callaghan (Kerry Gaelic footballer) (born 1987)Denis O'Callaghan (born 1949), Australian rules footballerDonncha O'Callaghan (born 1979), Irish international rugby union playerEdmund Bailey O'Callaghan (1797–1880), doctor, journalist, and leader of political movements in Ireland and QuebecFred Callaghan (born 1944), footballer with FulhamSir George Callaghan (1852–1920), officer in the Royal NavyGeorgina Callaghan, who performs as Callaghan, British musicianGuy Callaghan (born 1970), New Zealand butterfly swimmerJames Callaghan (1912–2005), British prime minister during the Winter of DiscontentJohn Carter Callaghan (1923–2004), Canadian cardiac surgeonJohn Callaghan (born 1960), Principal & Chief Executive Solihull College & University CentreJohn O'Callaghan (Medal of Honor) (1850–1899), American soldier and Medal of Honor recipientJohn O'Callaghan, Irish musician and DJJohn O'Callaghan, lead singer of The MaineJosé O'Callaghan Martínez (1922–2001), Spanish Jesuit Catholic priestJoseph T. O'Callahan (1905–1964), Captain, US Navy, Jesuit priest, Medal of Honor recipientLeanne Callaghan (born 1972), British ski mountaineer and mountain climberMary O'Callaghan, executiveMiriam O'Callaghan (camogie), 26th president of the Camogie AssociationMiriam O'Callaghan (media personality) (born 1957), Irish television current affairs presenter with RTÉMorley Callaghan (1903–1990), Canadian writer, playwright, and media personalityPatrick Callaghan (1879–1959), Scottish footballerSeán O'Callaghan (born 1954), from Tralee, County Kerry; IRA informerSheila Callaghan (born 1973), American playwrightSteve Callaghan, writer of Family Guy TV seriesStuart Callaghan (born 1976), Scottish footballerTherése O'Callaghan, a camogie player captainWilliam M. Callaghan (1897–1991), Admiral, US Navy; commander of the battleship MissouriO'Callaghans Mills, County Clare, IrelandCallaghan, New South Wales, a suburb of Newcastle, New South Wales and home of the University of Newcastle, AustraliaLiscallaghan, old Irish name for Fivemiletown, County TyroneCallaghan, Virginia, a census-designated place in the USUSS Callaghan (DDG-994)Mount Callaghan