The surname Mulcahy is of Irish Gaelic origin, being anglicised from "Ó Maolchathaigh". The Ó Maolchathaigh sept most likely originated in County Waterford, and 16th and 17th century records show that the family was most numerous in that county as well as Cork and Tipperary.
The surname refers to "a descendant of a devotee of Cathach"; the Cathach commemorated may refer to Saint Cadoc or Catwg, whose name is the Welsh equivalent of Cathach. It has been noted by such experts as Edward McLysaght that it is truly remarkable how little is heard of this family in any of the Medieval Annals or indeed by its modern representatives. The earliest possible reference to any bearer of the surname is to be found in The Annals of Innisfallen where an individual going by the name gilla moenaig omolchatha is mentioned in an entry for 1317AD.
There exists a popularly circulated theory regarding the Mulcahy families origins. The theory first proposed by John O'Hart claims that the Mulcahy family descend from the O'More of Laois. O'Hart makes mention of a John O'More who he claims was a younger brother of Rory Caech O'More and who adopted the surname Maolcatha. This theory is almost certainly wrong. O'Hart, intentionally or unintentionally had attributed the origins of the Mulcahy family with O'More on the basis of the name which John O'More had adopted. The name John O'More adopted was not Maolchatha as O'Hart had claimed but actually Maolchathail, and it is from him that the County Laois sept of Ó Maolchathail usually anglicized Mulhall, claim descent. To this day many commercial heraldic outlets continue to pass the traditional arms of the O'More family of Laois, which are "Vert a lion rampant or in chief three mullets of the last", off as those of Mulcahy when there is clearly no connection between the two families.
- Ballyogaha (Baile Uí gCathaigh), in County Cork, which may or may not have had some connection with the family
- Mulcahy Middle School, a middle school in the San Joaquin Valley in California
- Mulcahy Stadium, a baseball stadium in Anchorage, Alaska
- Alan Mulcahy (born 1983), Irish professional football player
- Anne M. Mulcahy (born 1952), American businesswoman, chairman and CEO of Xerox
- Dan Mulcahy (1882-1953), Australian politician
- John Duffy and David Mulcahy (born 1959), English rapists and murderers in the 1980s
- Denis Mulcahy (born 1956), Irish hurling player
- Francis P. Mulcahy (1894-1973), United States Marine Corps general and commander during World War II.
- Geoffrey Mulcahy (born 1942), British businessman with Kingfisher plc
- Hugh Mulcahy (1913–2001), American professional baseball player
- John Mulcahy (1876–1942), American Olympic double scull rower in the 1904 Olympics
- Kevin V. Mulcahy (born 1945), American professor and culture scholar
- Mark Mulcahy (contemporary), American jangle-pop musician in the 1980–90s
- Mark Mulcahy (contemporary), Movie Industry Executive 1981–Current
- Michael Mulcahy (born 1952), Irish expressionist painter
- Michael Mulcahy (born 1960), Irish Fianna Fáil party politician; TD for Dublin South-Central
- Pat Mulcahy (born 1975), Irish hurling player
- Richard Mulcahy (1886–1971), Irish politician, leader of Fine Gael and Cabinet Minister; Chief of Staff of the IRA
- Russell Mulcahy (born 1953), Australian film director
- Tomás Mulcahy (born 1963), Irish hurling player
- Thomas Michael Mulcahy (born Jan 1901), Secretary Canterbury Hospital and Australian Labour Party Member.
- Fictional person
- Father Mulcahy, character in the film and television show M*A*S*H
- E.MacLysaght, Irish Families, H&F Dublin 1957, p.232
- The Annals of Innisfallen, folio 57r
- J.O'Hart, Irish Pedigrees, Stem of the Irish Nation
- The general armory of England, Scotland, Ireland, and wales, p.715
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