What does the O in front of Irish surnames mean?

A male's surname generally takes the form Ó/Ua (meaning "descendant") or Mac ("son") followed by the genitive case of a name, as in Ó Dónaill ("descendant of Dónall") or Mac Lochlainn ("son of Lochlann"). A son has the same surname as his father. ... When anglicised, the name can remain O' or Mac, regardless of gender.