This is a surname of Irish origins. It derives from the medieval Gaelic Mac Raghallaigh and is said to loosely translate as "The son of the descendant of the rakish one". The majority of Irish clan names originate from a nickname for the first chief, from which it must be assumed that the original chief was a bit of a lad. There are two separate clans or septs which carry the name. Now spelt almost exclusively as Crilly, sometimes Crielly, Crelly, or Crolly, and just occasionally found with the O', Mac, or Mc prefix, the nameholders are found either in County Lowth in the Republic of Ireland or as McCroly in County Armagh in Ulster. The clan seem to have been prominent supporters of King James 11nd of England and king of Ireland from 1688 to 1690, as a number appear in the lists of attainted Jacobites in 1695, when they may well have lost their lands. What is certain is that several nameholders served in the the O'Neill regiments of the kings army, and after the defeat at the battle of the Boyne, moved to France. There they served in the Irish Brigades of the French forces. Early examples of the name recordings include James O'Crilly, a Jacobite officer in 1690, Patrick Crieely, who acted as a go between in negotiations between Charles 1st and Owen Roe O'Neill on behalf of Ulster Catholics in 1640,whilst Dr William Crolly was the bishop of Down in 1835.

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