"My mouth is red."
Translation:Tá mo bhéal dearg.
Dearg in this sentence is predicative, not attributive. Predicative adjectives aren’t declined. If dearg were attributive, it would only be lenited following béal (a singular masculine noun) if béal were genitive or vocative.
EDIT: An attributive dearg could optionally be lenited after béal if béal followed a leniting den, don, or sa, and it would be lenited if it were part of other non-eclipsing definite prepositional phrases — e.g. sa bhéal dearg, sa bhéal dhearg, sa mbéal dearg, ar an bhéal dhearg, and ar an mbéal dearg are all possible.