The surname Dempsey is generally accepted as being of Irish origins and therefore correctly O'Dempsey. There can be little doubt that most nameholders do derive originally from this source. However the name has also been long recorded in England James Dempse being recorded in the Guild of Corpus Christi, for the city of York, in 1526, and it is believed that English name holders derive from now "lost" medieval village called Demsey, meaning the island (eg) on the lake (dembel), however this is not proven. The Irish clan was once one of the most powerful in the country, possessing great estates in the area known as Clanmalier, now the modern counties of Leix and Offally. James 1st of England and V1 of Scotland granted Terence O'Dempsey the title of Viscount Clanmalier, for the support of the clan during the many rebellions of the 16th and 17th century. The name is an anglicised spelling of the Gaelic O'Diaomasach, which translates as 'The descendant of the proud one'. In 1170 the chief of the Dempsey's opposed the landings of Strongbow, Earl of Pembroke, and, it is said, was the only Irish commander to be successful in such engagements. At sometime in the following centuries the clan became loyal to the English crown, a loyalty that ultimately lost them all their lands when they supported the Catholic King James 11 at the battle of the Boyne in 1690. Early recordings of the surname include: Lewis O'Dempsey in 1652 in Dublin, Ireland, and in England Thomas Dempsey who was married at St Anne's church, Westminster, in 1692. The first recorded spelling of the family name is believed to be that of Dermot O'Dempsey, which was dated 1193, It is said that he was the founder of Monasterevan Abbey.

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