How about - they spoke fluent Irish or they were competent speakers of Irish ?

To be fluent doesn't mean you speak properly. You can be fluent and make 10 mistakes in a sentence. Fluent means you don't stop at every word to think about how you're going to say things But you can be fluent and speak poorly, if you've not learnt properly or from good speakers.
"Competent speakers" is what I meant, yes.
One could also say "native-like speakers"

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

And you're implying that such is the case with all or most native speakers who weren't raised by Gaeltacht speakers, which is patently false.

To me, someone raised by Gaeltacht speakers and someone raised by non-native speakers can't be compared except when the parents were native-like speakers (but I don't think there are loads of them: most of the time you just need to hear someone about 2 seconds to know if he/she is a native speaker or not, because the majority of non-native speakers use English languages sounds when speaking Irish). As for all languages, native speakers know more than learners (and they know things most learners can't know because they aren't written in books), and it's especially true in the case of minority languages, and even more in the case of Irish, since the natural language is almost never taught. To speak like a native speaker, you really need to spend much time in the Gaeltacht, and not many people do that. And also certain people go to the Gaeltacht but don't manage, or don't want, to speak like native speakers. But I think most learners would manage to speak native-like Irish if they worked hard enough. You don't need to be gifted to speak native-like Irish

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne