Irish family names reflect the long history of Ireland. During the time of the Celts and into the 11th century surnames were not passed down as they are today. A surname, as we know it today, may only have lasted a generation or two.
The prefix 'mac', meaning 'son of', was used. So Kieran mac Fiachra was Kieran the son of Fiachra, and his son might be Darragh mac Kieran i.e. Darragh the son of Kieran.
Alternatively, the prefix 'O' (or the older form 'Ua') was used to mean 'grandson of' or 'descended from'. So Kieran O'Conor was the grandson or descendant of Conor. While Oisín O'Laoghaire was a descendant of Laoghaire.
This echoing of the first name can still be heard today in many of the most instantly recognisable Irish last names, with or without Mac or O' prefixes, such as Connor/O'Connor, Cormack/MacCormack, Dermot/MacDermot, Leary/O'Leary.
This arrangement was considered entirely satisfactory until around the time the great Brian Boru became High King. When he was killed in 1014, he was known only as Brian. He is afforded a surname by history only.
When Brian Boru's grandson, Teigue, adopted Ua Briain to identify his descent from his heroic grandfather, he passed on the tag to his children and their descendants. Today, O'Brien is one of the top 10 most numerous surnames in Ireland.
When the Normans invaded in 1169 they brought with them Anglo-French names such as Burke, Costello, Nagle, Nugent, Power, Roche and Walsh. Burke came from de Burgh, Roche from de Roiste, Power from de Paor, while Fitz can be considered the continental version of Mac since it derives from the Latin/French fils de , meaning 'son of'.
During this period many of the medieval trades became incorporated into family names, such as Archer, Butler, Carpenter, Draper, Skinner, Tanner and Woodman.
In time these names evolved to fit the Irish language and then changed again during the 17th and 18th centuries when many where anglicised, thus obscuring the origins of these names.