The very first record of the family name McCormick was found in Munster, Ireland. The name McCormick in Ireland is derived from the native Gaelic Sept Mac Cormaic, which in turn is derived from the forename Cormac. The name is found mostly in Counties Fermanagh and Longford where the majority of descendants can still be found.
The 1890 index shows McCormick as the preferred spelling in the Ulster counties of Antrim and Down. McCormack is favored in Dublin, Mayo, Roscommon and Limerick. The shortened form of Cormack is found in Kilkenny and Tipperary in much smaller numbers.
In 1659 Cormac was a principal name of Kildare, O'Cormack a principal name of Cork and also found in Limerick, while McCormack was a principal name in Down also found in Cork, Kerry, Limerick and Roscommon.
McCormick at that time was found as a principal name of Antrim and in other Ulster counties. Origins for the name are unsure, and it probably was taken on by several unrelated familes at the same time. (i.e. McCormack simply means 'son of Cormac').
Counties Fermanagh and Longford show the name early as well, and a McCormack family of Cork (Muskerry) was of some note, also.
O'Cormacan is an example of another name which may have been changed into McCormack or Cormac. Cormican is found in Galway in the birth index as well, adding some confusion for these two names.
The name comes into prominence later than many others, and it is therefore thought that the name was taken on by families at a later date than most others.
Note the famous Irish tenor, John McCormack in America.
Births of the surname in 1890 (by province): Leinster 40, Munster 2, Connacht 16, Ulster 107
Most common in counties: Antrim, Dublin, Down.
Arms for the name are found on plates 238 and 264 in the Irish Book of Arms
Source: The Book of Irish Families Great & Small
Michael C. O'Laughin
1997 Irish Genealogical Foundation