Recorded in many spellings including Costello (County Mayo), Costelloe (Counties Galway and Clare), and sometimes in the dialectal variants Cassely, Cassley, Caserly, Casserly, Costley and Cusheley, this famous surname is Irish, but of Norman-French origins. It derives from the pre 10th century family known as the 'de Anglos' or Nangle, which strictly speaking means 'from or of, England'. It is claimed that the first reference to the clan in surviving registers or charters is that of Gilbert de Nangle in in the book known as 'The four masters' and dated 1193. This was only some twenty years after they helped Strongbow, Earl of Pembroke, in 1169 and 1170, conquer most of the island of Ireland. It seems that unlike most Norman families who were granted huge tracts of land by King Henry 11nd of England, they rapidly abandoned their Norman ancestry and became 'Gaelic'. Most Irish researchers claim that 'they saw the light', but it seems more logical that the clan saw the opportunity for even greater land gains by local banditry. Central government in Dublin was rarely strong in the next seven hundred years, and soon lost control of the hinterland to the west. The clan established itself at the barony of Costello in County Mayo, where for centuries they 'enjoyed' a local and bloody feud with the MacDermots. The first known holder of the surname is believed to be that of Cumumhan Mac Casarlaigh in about 1252, whilst in the 'modern' spelling it may be that of Teag MacCostello, chief of the clan in 1565. John Costello was Taoiseach of Ireland in 1948 and again in 1954.

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