Dempsey is a surname of Irish origin.
According to John Grenham:
"In the original Irish Dempsey is Ó Diomasaigh, from diomasach, meaning "proud". The name was also occasionally anglicised "Proudman". The Ó Diomasaigh originated in the territory of Clanmalier, on the borders of what are now counties Laois and Offaly, and remained powerful in the area until the seventeenth century. In the 12th century, Henry II set his court up in Dublin and summoned the Leinster Chiefs. The O’Dempsey, Chief of Offaly,refused to attend. Strongbow together with his son-in-law De Quincy marched into the O'Dempsey territory to take land by force. In a famous battle the O'Dempsey together with his followers routed the Normans with great slaughter, an action that kept the Normans out of the Dempsey territories for over 300 years. In later years, their allegiance was to the English and they were involved with the newcomers in the massacre of the O’Lalors in Laois in 1577, an action which local tradition says was responsible for their later losses. Charles the first recognised the strength of the family by granting the English titles of Viscount Clanmaliera and Baron Philipstown in 1631 PJ Goode - The O'Dempsey Chronicles>to Terence Dempsey. The loyalty of the family to the Crown was short-lived, however, and the Williamite wars later in the century destroyed their power and scattered them." 
Another source states:
"The O'Dempsey family derive their name from Diummasach, an 11th-century Uí Failge prince of the Clann Máel Ugra, aka Cenél Maoilughra. The Clann Máel Úgra, in turn, took their name from Máelaugrai, an Uí Failge chieftain who flourished in the middle of the 9th century. The center of their territory, anglicized Clanmalier, was near Ballybrittas in northeast Co. Leix. Their power later extended into the barony of Upper Phillipstown. O'Donovan, in his Ordnance Survey letters, places Clann-Maoilughra in the present Barony of Upper Philipstown in the King's County and the Barony of Portnahinch in the Queen's County." 
The family of the Viscounts Clanmalier upon the death of Maximilian, the 3rd Viscount, in 1691, were scattered largely destitute and Terence O'Dempsey fled Ireland along with his cousin Sir Peter Byrne to Cheshire where the core family flourished. The original velum Charles 1st Title still exists.
An Ó Diomasaigh genealogy records the following:
Flann m. Máel Ruanaid m. Cellaich m. Máel Augra m. Conchobuir m. Áeda m. Tomaltajich m. Flaind m. Díumasaich m. Congaile m. Forannáin m. Congaile m. Máel h-Umai m. Cathail m. Bruidge m. Nath Í m. Rosa Failgi. 
A new bloodline has been recently traced to when the Dempsey's came to America. When the family known as the Dempseys got off the boat, they were asked their legal names by American officials, who would put them in the American records, making them a citizen, it was misspelled Dimsey, therefore creating the Dimsey bloodline. Another group of the family came on the boat, and was correctly identified as Dempsey, leading to the American Dempsey bloodline.
Dempseys in the annals
The Irish annals list a number of members of the family:
- 789: Áedh [grandfather of Máelaugrai] was slain by Óengus son of Mugrón, king of Uí Failge, in the oratory of Kilclonfert.
- 1141: Donnchadh, son of Goll Gaibhle, i.e. Ua Conchobhair Failghe, was killed by the Ui-Failghe themselves, i.e. the Clann-Maelughra.
- 1161: Domhnall, son of Conghalach, son of Cuaifne Ua Conchobhair Failghe, Tanist of Ui-Failghe, was slain by the Clann-Maelughra.
- 1164: Maelseachlainn Ua Conchobhair Failghe, was slain by the Clann-Maelughra.
- 1193: Diarmait, son of Cubrogam Ua Diumasaigh, chief of Clann-Mailighra and king of Ui-Failghe for a long time, died.
- 1383: Dermot O'Dempsy, Lord of Kinel-Maoilughra, was slain by the English.
- 1394: Thomas O'Dempsy, heir to the lordship of Clann-Maoilughra, was slain by the English.
Other Dempsey families
Bearers of the name
The name Dempsey may refer to:
- Tanya Rose Dempsey, World Class Painter (1977-)
- Muriel Dempsey (1905–1986), Canadian author
- Rosemary Dempsey, American feminist activist
- Sandra Dempsey, Canadian playwright
- Bill Dempsey (born 1942), former Australian rules footballer
- Cedric Dempsey, former American sports executive
- Chester Dempsey (1896–1969) American farmer and politician
- Clint Dempsey (born 1983), American footballer
- Courtenay Dempsey (born 1987), Australian rules footballer
- Damien Dempsey, Irish singer and songwriter
- Dan Dempsey, Australian rugby league footballer
- Denis Dempsey (1826–1896), Irish soldier, recipient of the Victoria Cross
- Donald Dempsey (c. 1932–2005), American recording executive
- Ed Dempsey, Canadian ice hockey coach
- Frank Dempsey (1925–2013), American football player
- Gary Dempsey (disambiguation), multiple people
- George Dempsey (disambiguation), multiple people
- Girvan Dempsey (born 1975), Irish rugby union footballer
- Ian Dempsey (born 1961), Irish radio presenter
- Jack Dempsey (disambiguation), multiple people
- James Dempsey (1917–1982), Scottish politician, MP 1959–1982
- John Dempsey (disambiguation), multiple people
- Lorcan Dempsey (born 1958), Irish library information expert
- Mark Dempsey (disambiguation), multiple people
- Martin Dempsey (born 1952), American general, 18th and current Chairman, US Joint Chiefs of Staff
- Michael Dempsey (disambiguation), multiple people
- Mick Dempsey (Gaelic footballer), retired Irish gaelic footballer
- Miles Dempsey (1896–1969), British general of World War II
- Nathan Dempsey (born 1974), Canadian ice hockey player
- Nick Dempsey (born 1980), English windsurfer
- Noel Dempsey (born 1953), Irish politician
- Patrick Dempsey (born 1966), American actor in Grey's Anatomy, movies
- Paul Dempsey (born 1976), Australian rock musician
- Rey Dempsey, retired American college football coach
- Richard Dempsey (born 1974), English actor
- Rick Dempsey (born 1949), retired American baseball player
- S. Wallace Dempsey (1862–1949), American Republican politician, Congressman 1915–1931
- Thomas Dempsey (disambiguation) or Tom Dempsey, multiple people
- Tommy Dempsey, American college basketball coach
- Tony Dempsey (born 1944), Irish Fianna Fáil politician, TD 2002–2007
- Travis Demsey, born Travis Dempsey, Australian musician
As a given name:
- Dempsey Bob (born 1948), Canadian woodcarver
- Dempsey Burges (1751–1800), American politician, Congressman 1795–1799
- Dempsey Wilson (1927–1971), American racecar driver
- P. Dempsey Tabler (1876–1956), American actor, athlete and businessman
- Dempsey Taratoa Park (born 1991), New Zealand Maori activist
- P.H. Reany & R. M. Wilson, A dictionary of English Surnames, Oxford University Press, Oxford, 3rd ed., 1995, p. 131a; Edward MacLynsaght, The Surnames of Ireland, Irish Academic Press, 6th ed., Dublin, 1991, p. 79.
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