Last names and Irish places
Gaelic prefixes, anglicised spellings and the counties where names cluster.
Irish surnames are among the oldest in Europe, with some in use by the 10th century. Most began as Gaelic names and were respelt in English over several centuries.
O and Mac
Ó (O') means "descendant of" and Mac means "son of": O'Brien (Ó Briain), Murphy (Ó Murchadha), McCarthy (Mac Cárthaigh). During English rule the prefixes were often dropped, and many families later restored them, so a family may appear with and without O' in different records.
Anglicisation
Gaelic names were turned into English forms by sound (Ó Murchadha to Murphy) or by translation (some names that meant a colour or trade were rendered into the English word). Because both happened, the same Gaelic name can have several English forms, and one English name can come from several Gaelic ones.
Names and counties
Many names are linked with a region, such as Sullivan in Cork and Kerry, O'Neill in Ulster, Kelly in Galway, Roscommon and Tyrone. These are tendencies from historical records and the Griffith's Valuation of the 1850s; they are not proof of descent. Planter families from Scotland and England also brought names such as Campbell, Wilson and Johnston to Ulster.
Records
Much of the 1901 and 1911 census is free online at the National Archives of Ireland. Civil registration began in 1864 (non-Catholic marriages from 1845). Church registers go back further for some parishes. Griffith's Valuation lists householders and landholders in the 1840s and 1850s.