This interesting surname is of Irish origin, and is an Anglicized form of the Gaelic "O'Raghailligh", descendant of Ragheallach. The O'Reillys are thought to be kinsmen of the O'Conor kings of Connacht through Maolmordha, whose great grandson was Ragheallach, from whom the O'Reillys took their name. Traditionally, Irish family names are taken from the heads of tribes or from some illustrious warrior, and are usually prefixed by "O", grandson, male descendant of, or "M(a)c" denoting "son of". The O'Reilly territory was around Lough Oughter in Cavan and as they increased they extended their families and their territory, particularly to Westmeath. Cavan's Franciscan Abbey was founded by Giolla Iosa O'Reilly. There were 39 O'Reilly abbots and five held the primacy of Armagh as well as several bishoprics. They were also kinsmen of St. Oliver Plunkett. The celebrated Count Alexander O'Reilly from Meath distinguished himself first in the Austrian service and then in the Spanish army. He put down a rebellion in Louisiana where he became governor. The O'Reillys have a reputation as astute financiers. In the 15th Century they devised their own coinage. A "Reilly" has come to signify a coin of useful value. The Coat of Arms most associated with the family is a green shield with two gold lions rampant combatant, supporting a dexter hand couped at the wrist erect and a paumee bloody proper. The first recorded spelling of the family name is shown to be that of Cathal O'Reilly, Prince of Breffny, which was dated 1237, in the "Records of Lough Oughter Monastery", County Cavan, during the reign of King Henry 111 of England, 1216 - 1272. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.

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