Scots - Irish Clans and Families - Dempsey

http://www.electricscotland.com/webclans/scotsirish/dempsey.htm

Dempsey

The Dempsey name, which is also found spelt as Dempsie, Dimpsay or Dimpsey, is of Irish origin from the Irish 
Gaelic o'dimpsaigh meaning "estimable" or "son of the Proud". The Dempseys were descended from the 
O'Dempsey family, Clan Maliere.

O'Dempsey, or O'Dimpsaigh the old name that was translated "as strong as a rock" "estimable" or "son of the 
Proud" in ancient Ireland, suggests the eminence of the name. The O'Dempsey family was reputedly founded 
by Rossa Failghe, the son of Cathoir Mor, King of Ireland in 144 AD.

The O'Dempseys were of the same descent as the O'Connors Failghe, being lords of Offaly (a county in central 
Ireland) and sometimes styled Barons of Phillipstown (Daingean, Co Offaly, the former seat of the O'Connors). 
The O'Dempseys, also referred to as the Clan Maliere, were for long one of the powerful families in the Province 
of Leinster. Their estates in Clan Maliere extended on both sides of the River Barrow in Queen's County 
[now County Laois or Laoghis] and King's County [now County Offaly]. The lands comprised the baronies of 
Portnalinch in Laois and Upper Phillipstown in Offaly.

http://www.rootsweb.com/~irlkik/ihm/irenames.htm

Old Irish-Gaelic Surnames 

Mac Colghain - (Kil) or (Mac) Colgan - Co Offaly - same stock as O'Dempsey and O'Dunne