The Ballycastle Blog: Irish Genealogy and More

Many Irish surnames begin with O’, such as O’Grady, O’Sullivan, O’Neill, O’Reilly, and O’Brien. An O prefix on a surname translates to “grandson of”, whereas a Mac prefix translates to “son of”. It was not uncommon for Irish immigrants coming to America in the 19th century to lose the O’ or Mac, when they processed through U.S. Immigration, whether by choice or not. When searching for an Irish ancestor’s records in Ireland, it is wise to not only look for the surname without the O’, but also the surname with the O’. For example, in addition to looking for your grandfather James Sullivan in Ireland, you should also look for James O’Sullivan. You may find that your Irish family name originally had an O’ prefix long ago, before your ancestors traveled to America.

See more Irish family history articles and lessons learned in earlier posts below and in the archives.