When the sparse Irish population began to increase it became necessary to broaden the base of personal identification by moving from single names to a more definite nomenclature. The prefix MAC was given to the father's christian name, or O to that of a grandfather or even earlier ancestor. At first the coat of arms was a practical matter which served a function on the battlefield and in tournaments. With his helmet covering his face and armour encasing the knight from head to foot, the only means of identification for his followers, was the insignia painted on his shield and embroidered on his surcoat, the draped and flowing garment worn over the armour. Descendants of the brehon family that were so called O'Brisleain, none of whom seemed to have reassumed the prefix '0'. County Donegal is their region and they have always been associated with that area. The maritime Ulster county of Donegal in the extreme north-west of Ireland is bounded on the west and north by the Atlantic ocean, to the south by Donegal Bay and an extending extremity of County Leitrim on the east by Lough Foyle which separates it from County Derry and to the south-east by land boundaries with county Tyrone and County Fermanagh. The ancient name of the region was Tyrconnell or Tirconnell and its chief families were the ruling O'Donnells and O'Dohertys. The county was erected by the Lord Deputy in 1584, and after the forfeiture to the Crown of the O'Donnell estates, the lands of the county were included in the ambitious Ulster plantation scheme. About four-fifths of the cultivable land in the county was allotted for settlement in 62 portions, 47 for English and Scottish undertakers and servitors, and 15 for native Irish. The rest of the good land was assigned to the established church for its support to Trinity College, and for the support of schools in Derry and Donegal and to five corporate towns. The behons were the judges who referred to the Gaelic legal system, in force before the Norman Invasion.
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.
The name is also spelt O'Breslin.
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