McCormack or MacCormick are very common surnames in Ireland and are distributed widely throughout the four provinces.

There was a minor sept of the name MacCormack in County Longford, but for the most part the name appears to have come into existence independently in many places at a comparatively late date. Individuals whose fathers' christian name happened to be Cormac began describing themselves as MacCormaic which thereafter was continued as a surname by subsequent generations. MacTeigh and MacShane are other examples of this tendency.

In 1576, 1598 and again in 1600, MacCormacks are recorded as leading gentry in County Cork and one, of Muskerry, was influential enough to raise a large force to assist Desmond in the Elizabethan wars. The Four Masters record the deaths of several prominent MacCormacks of Fermanagh; the last of these died in 1431.

Possibly the MacCormacs of County Armagh were descendants of these: two of them were very prominent in the medical profession: Henry MacCormac (1800-1886), and his son Sir William MacCormac (1836-1901); while a third medical man, Robert MacCormack (1800-1890), best known as an Arctic navigator, was of Tyrone parentage. The name is universally recognized as Irish on account of the fame of John Count MacCormack (1885-1945), the tenor.

MacCormack has also been adopted in place of the surnames O'Cormack and O'Cormacan, borne by small septs located in Counties Roscommon, Galway, Clare, Cork, Down and Derry. Thus even when MacCormack is a substitute for those names it is very widely scattered. Three of the name O'Cormacan were Bishops of Killaloe in the thirteenth and fourteenth centuries.

Courtesy of:
Robert Bell's "Book of Ulster Surnames"