The tradition of surnames in Ireland developed spontaneously, as the population increased and the former practice, first of single names and then of ephemeral patronymics or agnomina of the nickname type proved insufficiently definitive. At first the surname was formed by prefixing 'Mac' to the father's Christian name or 'O 'to that of a grandfather or earlier ancestor. CALLAHAN families were descended from Ceallachain, king of Munster who died in AD952. The chief family were transported to Clare during the Cromwellian regime. The ancient territory of the sept was in the barony of Kinlea in the south of County Cork whence they moved northwards in the 13th century and resettled in the neighbourhood of Mallow in the same county. Their descendants are still largely to be found in the area of County Cork. Ireland was one of the earliest countries to evolve a system of hereditary surnames. They came into being fairly generally in the 11th century, and indeed a few were formed before the year 1000. Many factors contributed to the establishment of a surname system. For generations after the Norman Conquest of 1066 a very few dynasts and magnates passed on hereditary surnames, but the main of the population, with a wide choice of first-names out of Celtic, Old English, Norman and Latin, avoided ambiguity without the need for a second name. As society became more stabilized, there was property to leave in wills, the towns and villages grew and the labels that had served to distinguish a handful of folk in a friendly village were not adequate for a teeming slum where perhaps most of the householders were engaged in the same monotonous trade, so not even their occupations could distinguish them, and some first names were gaining a tiresome popularity, especially Thomas after 1170. The hereditary principle in surnames gained currency first in the South, and the poorer folk were slower to apply it. By the 14th century however, most of the population had acquired a second name.
The associated coat of arms is recorded in Sir Bernard Burkes General Armory. Ulster King of Arms in 1884. Registered in Ireland.
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