The issue with this is the "dentals" rule. Normally, it wouldn't lenite. However, since deacra is a attributive adjective, it would still lenite, despite the rule. Yet, even in native speech, it doesn't always do this. So really, you can use either form in these types of sentences, though the most "correct" form is to always lenite attributive adjectives.
(This is the difference between things like Rinne muid an ceist deacair (we made the question hard) and Rinne muid an ceist dheacair (We made the hard question))