To say "black hair" in Irish, you use the phrase gruaig dhubh.
The Irish word dubh (pronounced 'doov') means 'dark' or 'black'. This term is widely used to describe hair color and has a historical presence in Irish names to signify a person's dark features, as seen in Róisín Dubh (Dark Rosaleen) or Aodh Dubh Ó Néill (Black Hugh O'Neill). In Irish grammar, adjectives typically follow the noun, and the initial consonant of the adjective may change (a process called lenition) depending on the preceding noun's gender and case. In the case of "gruaig dhubh," 'gruaig' is a feminine noun, causing 'dubh' to lenite to 'dhubh'.
