In the original Irish Dempsey is O Diomasaigh, from diomasach, meaning "proud". The name was also occasionally anglicised "Proudman". The O 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. O?Dempsey, Chief of Offaly, was one of the few native Irish leaders who defeated Strongbow. 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. James I recognised the strength of the family by granting the title "Viscount Clanmalier" 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. The surname is now found throughout the country. It is recorded in the placename Knocknadempsey, in Neddans civil parish in south Tipperary. In Ulster, Dempsey is common in Co. Antrim, where it may be a version of "Dempster", a Scottish name meaning "judge", or possibly an anglicisation of Mac Diomasaigh, also sometimes rendered as "McGimpsey".
112 births of the name are recorded in 1890, with particular concentrations in Antrim, Cork and Wexford, making it the 192nd most common surname in the country; by 1996 it had risen to 164th.
.Jeremiah Dempsey (1906 - )
provided the guiding spirit behind the Irish national airline, Aer Lingus, from its foundation in 1937 until his retirement in 1967. He was President of the IATA in 1962.
Noel Dempsey T.D. (1945 - ) has been a prominent member of the Fianna Fail political party for many years, and minister of state in number of governments.
William Harrison Dempsey (1895-1983), born in Manassa, Colorado isbetter known as Jack Dempsey, one of the most popular heavyweight boxing champions of all time. His ferocious attacking style earned him the nickname "The Manassa Mauler". He was world champion from 1919 to 1926, when he lost the title to Gene Tunney.