gruaig fhionn - - fair hair

fionn is an adjective meaning fair.
Here the f of fionn is "aspirated" i.e. fh showing that the word gruaig, hair, is feminine.

Aspiration or lenition is called "séimhiú" in Irish and is a distinctive feature of the language.

fear fionn a fair-haired man is an example of the adjective with a masculine noun.