What does the mean in Irish surnames?

Ó or Ua - the most common prefixes in Irish surnames, mean male descendant. For example, Ó Cathasaigh (Male descendant of Cathasaigh)

Ní - female decendant. For example, Ní Chonaill (Female descendant of Conall)

Mac - son of or male child. For example, Mac Mathúna (Son of Mathún). This prefix can be used whether or not the surname is a patronymic.

Uí - descendants. For example, Uí Bhriain (descendants of Brian)