- A prefix used with a woman's maiden surname
- Deirdre Ní Cheallaigh – literally "Deirdre, daughter of a descendant of Ceallach" (anglicized form Deirdre Kelly)
This prefix replaces the prefix Ó found in male surnames, and triggers lenition of a following consonant. It is used by unmarried women, and by married women who have kept their maiden name. Thus Deirdre Ní Cheallaigh's father and brothers would use the surname Ó Ceallaigh.