William Callahan

(male) Probably the most successful of all the Old French names of Germanic origin that were introduced to England by the Normans. It is derived from Germanic wil ‘will, desire’ + helm ‘helmet, protection’. The fact that it was borne by the Conqueror himself does not seem to have inhibited its favour with the ‘conquered’ population: in the first century after the Conquest it was the commonest male name of all, and not only among the Normans. In the later Middle Ages it was overtaken by John, but continued to run second to that name until the 20th century, when the picture became more fragmented.

Irish: reduced Anglicized form of Gaelic Ó Ceallacháin ‘descendant of Ceallachán’, a diminutive of the personal name Ceallach, possibly meaning ‘lover of churches’, from ceall ‘church’, or (more likely) ‘bright-headed’, from cen ‘head’ + lach ‘light’. This name was borne by a 10th-century king of Munster, from whom many present-day bearers of the surname claim descent.