What is the difference in meaning between "deá-" and "maith"? The same goes for "droch-" and "dona" which both seem to mean 'bad' in English. Aside from the fact that "deá-" and "droch-" are prefixed adjectives what differentiates them respectively from "maith" and "dona" if anything at all? Are they perhaps interchangeable synonyms?

Thanks !!!

as you have pointed out, you can't use dea- or droch- except as prefixes. So they are not interchangeable.

Also dona and especially maith have wider meanings.

droch- [réimír]
olc, dona; gránna, suarach.

dona [aidiacht den tríú díochlaonadh]
mí-ámharach; i ndrochchaoi, olc; i ndrochshláinte.

Maith is also a noun.

maith [aidiacht den chéad díochlaonadh]
cairdiúil, cneasta, cineálta (comharsa mhaith é); dea-iomprach (páistí maithe); den chineál ceart (talamh maith); sásúil, folláin (bia maith); fónta (peann maith); cumasach (oibrí maith); ceart, cóir (cúis mhaith); oiriúnach (lóistín maith); muiníneach (cuimhne mhaith); fabhrach, taitneamhach (scéala maith); mór, fiúntach (suim mhaith airgid); láidir, tréan (greasáil mhaith); (mar fhocal treisithe) (baile maith mór, anonn go maith sa bhliain).

maith [ainmfhocal baininscneach den dara díochlaonadh]
rud maith, gníomh fónta; (le buíochas) (go raibh maith agat).

dea- [réimír]
maith.