barbarian would depend on what you mean by Barbarian

-could be literally, 'hairy' or 'gruagach', maybe even 'bearded' which might be created as 'féasógach' from 'féasóg', though having a beard or hair does not necessarily have the same implications it once did! though the pejorative 'long-hair' as a synonym for 'hippy', 'dirty', etc., from the 60s might apply

-could be 'Sasanach', somewhat perjorative for 'Englishman', and in some contexts 'uncivilized', etc.

-could be 'un-Christian', or 'ní Críostaí'

-could be 'ruthless', or 'neamhthruacánta'

-could be 'uncivil', or 'ní sibhialta'

-could be 'stupid', or 'amaideach'

-could be 'ignorant', or 'aineolach'

....