Names formed in Irish, from a real ancestor — the oldest hereditary surnames in Europe.
Ireland took to hereditary surnames earlier than most of Europe: Ó Cléirigh, anglicised Cleary and Clarke, is described as one of the oldest recorded surnames on the continent. The form is a genealogy in two words. Ó means grandson or descendant, Mac means son, and what follows is the personal name of a real ancestor — so Ó Murchadha is the descendants of a man called Murchadh, and Ó Ceallaigh the descendants of a Ceallach. The group of families claiming that descent was a sept, and a sept held land in a particular district, which is why every name on this site comes with homeland counties.
Not every name is built on an ancestor. Some are built on a job that ran in the family: the Ó Dálaigh were poets, the Ó Cléirigh chroniclers, the Mac an Bhaird, Ward, literally the sons of the bard, and the Ó hÍcidhe, Hickey, the physicians. These hereditary learned families served the Gaelic lords for generations and are marked here as learned families rather than septs. Others are built on devotion: the Giolla and Maol names (McElroy, Mulholland, Molloy) mean the servant or the devotee of a saint or a quality.
The English forms came later, and by ear. From the sixteenth century the names were written down by English-speaking clerks as they heard them, which is how one Irish name can arrive in English as Kehoe and Keogh, or Sheridan and O'Sheridan. Through the eighteenth and nineteenth centuries the Ó and Mac were widely dropped, and from the late nineteenth century, with the Gaelic revival, widely put back. O'Connor and Connor, Sullivan and O'Sullivan, are the same name at different moments in that story.
Sources: Wikipedia — Irish name ↗ · Wikipedia — Ó Cléirigh ↗
Homelands are the historic heartland of the name, not where the family is now — people moved, and the Famine and the plantations moved them further.
Ó Cléirigh, anglicised Cleary and Clarke, is described as one of the oldest recorded surnames in Europe. Irish hereditary surnames as a system are older than those of most of the continent.
Ó means grandson or descendant of; Mac means son of. Both are followed by a real ancestor's personal name, so Ó Briain is the descendants of Brian and Mac Cárthaigh the sons of Cárthach. Mac names are in general a little younger than Ó names, since they take the ancestor's own son as the founder.
They were dropped. From the sixteenth century the names were written down in English by ear, and through the eighteenth and nineteenth centuries the prefixes were widely abandoned; from the late nineteenth century many families put them back. Sullivan and O'Sullivan are one name.
416: Adams, Agnew, Ahern, Bannon, Beattie, Begley, Behan, Bergin and more.
Most often Cork (84), Galway (74), Clare (59), Mayo (56).