This name is from the gaelic O'Monaghan. Given to the chief of the Hy Briuin na Sinna, a district in County Roscommon. The chief had his seat there in 1249. This became the Lordship of the O'Breirnes when O'Monaghan became chief of the three Tuathas in the same county. Families bearing the surname Mona(g)han, now found in all four provinces, descend from a sept O'Manachain, whose territory lay in northern County Roscommon along the borders of the baronies of Roscommon and Boyle. 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. Surnames as we know them today were first assumed in Europe from the 11th to the 15th Century. They were not in use in England or in Scotland before the Norman Conquest, and were first found in the Domesday Book. The employment in the use of a second name was a custom that was first introduced from the Normans. They themselves had not long before adopted them. It became, in course of time, a mark of gentler blood, and it was deemed a disgrace for gentlemen to have but one single name, as the meaner sort had. It was not until the reign of Edward II (1307-1327) it became general practice amongst all people. 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.
Most bearers of this name are descended from Manachan, a 9th century chief in Connaught.
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