The website Irish Central has posted a list of the 10 most popular Irish last names, including meaning of the name, variations, and the area where the name is most prominent.
The site also includes links to heraldry and history of the names.
Topping the list is Murphy:
This surname, which means "sea battler," translates to Gaelic as MacMurchadh (son of Murchadh) and O'Murchadh (descendent of Murchadh), a derivation of the first name of Murchadh or Murragh.
O'Murchadh families lived in Wexford, Roscommon and Cork, in which county it is now most common, with the MacMurchadhs of the Sligo and Tyrone area responsible for most of the Murphys in Ulster.
The name was first anglicized to MacMurphy and then to Murphy in the early 19th century.