The prefix Mac or Mc means “son of.” People usually associate Mac names with Scotland, but there are many in Ireland, too. The Mac generally precedes the name of the person's clan, which was one of the primary social units for the Celtic populations in both Scotland and Ireland. These surnames, like O' surnames, sometimes drop their prefix. So if, for example, your name is McMahon, you might have to research it under Mahon.
The name McBride and its relative Kilbride come from Gaelic words meaning “devotee of the cult of St. Brigid.” Brigid was one of Ireland's three patron saints, and her shrine was located at Kildare; there was a shrine to a different St. Brigid in Donegal, and many followers of the cult associated with it took the name McBride for themselves and their children. McBrides were very common in County Down in the 1600s. The St. Brigid cult was also popular in Scotland, which was the origin of many of the Kilbrides and the Bridges, some of whom moved to Ireland.
McCarthy is among the ten most common Irish surnames; the majority of them live in County Cork. The McCarthys trace their ancestry to the third-century king of Munster, Oilioll Olum, and his son Eoghan, founder of the Eóghanacht. The name comes from an eleventh-century ruler called Cárthach (which means “the loving one”); his descendants took the name McCárthach. McCarthys moved into Kerry, west Cork, and in the barony of Muskerry, where the McCarthys lived in Blarney Castle.
Irish women have their own family name system; Irish puts a Ní in front of an unmarried woman's surname, and an Uú in front of the surname of a married woman. Girls were often named Mary (or Maura), or after a female saint. Colleen isn't actually a common name for women; it's the Anglicized version of the Irish word for girl, cailín.
The name MacGuiness and variants originated in Ulster, where it is traced back to a fifth-century chief named Saran. In the twelfth century, the MacGuinesses controlled most of County Down. Many of them converted to Protestantism before the plantation.
In the late sixteenth century, though, the head of the family joined the O'Neills in their rebellion against the British, and the MacGuinesses found themselves dispossessed. Many of them fled to Europe, where they assisted France, Spain, and Austria in their fights against the British.
The three most common surnames in Ireland are Murphy, Kelly, and O'Sullivan. Murphy is the most common; Murphys are sometimes called “Spud Murphy,” because they are “as common as potatoes.”
McKenna is a common name in County Monaghan. It comes from a County Meath family called Mac Cionaith, who moved to Ulster to serve as soldiers. The McKennas were dispossessed after Cromwell, and many of them moved to Counties Derry and Down. Some McKennas changed their name to McKinney, though most Ulster McKinneys are descended from Scots with that name.
The McMahons (and MacMahons and Mahons) trace their ancestry to a grandson of Brian Boru, Mahon (“son of a bear”) O'Brien. The McMahons controlled the baronies of Moyarta and Clonderlaw in southwest County Clare; McMahon is now the most common surname in County Clare. Also, MacMahon is one of the top five names in County Monaghan.