First found in counties Cork and Mayo. It is thought that immigrants from Waterford took on this name after they arrived in their new homelands. First recorded instance of the name appears to be a bearer of Dacy, who was a Drogheda juror in Cork as early as 1313. There is a legend in Cork that all the Deasys of the county are ancestors of a child who was rescued from a massacre in the barony of Decies in about 1620. It is claimed that he was then protected by the O'Donovans of the Island, and went on to have seven sons.

Some of the first settlers of this family name or some of its variants were: Catherine Deasy, who arrived at St. John, New Brunswick in 1834; Martin Deasy, whose Oath of Allegiance was recorded in Philadelphia in 1835; Michael Deasy, who died on route to Quebec or at Grosse Isle in 1847.

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