Ró‑ is a prefix (thus, the hyphenation is required) that means too / excessively; also most (in the sense of 'it is most likely', 'the most holy', 'the most exhalted' ⁊rl…). It lenites, when followed by a lenitable consonant.
Dona is an adjective that means bad.
Thus, ró-dhona: too bad. (In the sense of 'too bad to put up with', 'too bad to salvage', 'too bad to use' ⁊rl… It is not suitable as a gaelicisation of the American English phrase too bad, in the sense of 'unfortunate', 'a pity' ⁊rl…)