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 footballer
Tabby Callaghan (born 1981), Irish guitarist and singer-songwriter
Barry O'Callaghan (born 1969), CEO of Riverdeep Interactive Learning
Bill O'Callaghan (1868–1946), Irish hurler
Billy O'Callaghan (born 1974), Irish short story writer
Con O'Callaghan (decathlete) (born 1908), Ireland's first Olympic decathlete; brother of Dr. Pat O'Callaghan
Christopher Callaghan, 2006 Republican candidate for New York State Comptroller
Daniel J. Callaghan (1890–1942), Rear Admiral, US Navy
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
Fred Callaghan (born 1944), footballer with Fulham
Sir George Callaghan (1852–1920), officer in the Royal Navy
Georgina Callaghan, who performs as Callaghan, British musician
Guy Callaghan (born 1970), New Zealand butterfly swimmer
James Callaghan (1912–2005), British prime minister during the Winter of Discontent
John Carter Callaghan (1923–2004), Canadian cardiac surgeon
John Callaghan (born 1960), Principal & Chief Executive Solihull College & University Centre
John O'Callaghan (Medal of Honor) (1850–1899), American soldier and Medal of Honor recipient
John O'Callaghan, Irish musician and DJ
John O'Callaghan, lead singer of The Maine
José O'Callaghan Martínez (1922–2001), Spanish Jesuit Catholic priest
Joseph T. O'Callahan (1905–1964), Captain, US Navy, Jesuit priest, Medal of Honor recipient
Leanne Callaghan (born 1972), British ski mountaineer and mountain climber
Mary O'Callaghan, executive
Miriam O'Callaghan (camogie), 26th president of the Camogie Association
Miriam O'Callaghan (media personality) (born 1957), Irish television current affairs presenter with RTÉ
Morley Callaghan (1903–1990), Canadian writer, playwright, and media personality
Patrick Callaghan (1879–1959), Scottish footballer
Seán O'Callaghan (born 1954), from Tralee, County Kerry; IRA informer
Sheila Callaghan (born 1973), American playwright
Steve Callaghan, writer of Family Guy TV series
Stuart Callaghan (born 1976), Scottish footballer
Therése O'Callaghan, a camogie player captain
William M. Callaghan (1897–1991), Admiral, US Navy; commander of the battleship Missouri
O'Callaghans Mills, County Clare, Ireland
Callaghan, New South Wales, a suburb of Newcastle, New South Wales and home of the University of Newcastle, Australia
Liscallaghan, old Irish name for Fivemiletown, County Tyrone
Callaghan, Virginia, a census-designated place in the US
USS Callaghan (DDG-994)
Mount Callaghan
