For other uses, see Dempsey (disambiguation).

Dempsey is a surname of Irish origin.


Dempsey is an anglicised form of Ó Diomasaigh, " descendant of Díomasaigh "; the latter stems from the Irish adjective díomasach " proud ".[1] The family originated in the Kingdom of Uí Failghe.

According to John Grenham:

"In the original Irish Dempsey is Ó Diomasaigh, from diomasach, meaning "proud" or "arrogant". 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. Sir Terence: son of Dermod Ruadh; was knighted in May 1599 for services to the Crown by Robert Devereux, earl of Essex, lord lieutenant of Ireland; Charles the first recognised the strength of the family by granting the English titles of Viscount Clanmaliera and Baron Philipstown in 1631 to Terence O'Dempsey. (PJ Goode – The O'Dempsey Chronicles) 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 O'Dempsey, 3rd Viscount Clanmalier in 1691, were scattered largely destitute and Terence O'Dempsey the younger son of Maximilian fled Ireland along with his cousin Sir John Byrne to Cheshire ( Sir John married Merial Leicester, the heiress of Tabley Hall near Northwich) where the core family flourished. The original velum Charles 1st Title still exists along with the Ulster King of Arms Pedigree produced in 1830.

A direct descendent of Terence O'Dempsey was the WW2 General Sir Miles Dempsey who commanded the British 2nd Army at D Day.


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.

The final person may be identical with Failge Berraide (fl.507–514), a King of the Uí Failge and of Laigin descent (see List of kings of Uí Failghe).

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:

Other Dempsey families

An unrelated family, Mac Diomasaigh, are found in County Antrim and its neighbouring counties .

Bearers of the name

The name Dempsey may refer to:



As a given name:

External links


  1. 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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