Fòram na Gàidhlig

I thought I saw this on the Akerbeltz site, but I can't find it now.

Supposedly there's a difference (presumably subtle) between unvoiced consonants and devoiced consonants.

Can anyone tell me what it is?

There's none. It's just linguistic nit-picking over whether a word like bad has two voiceless stops i.e. /p/ and /t̪/ or whethere they are devoiced /b̊/ and d̪̊/. The first might be closer to what a neutral (i.e. one who doesn't 'do' Gaelic) phonetician might transcribe, the latter what a more Gaelic oriented phonetician might transcribe due to the historical devoicing of these sounds and some other marginal features which make this approach a bit more ... workable.

What comes out of your mouth is, to all intents and purposes, the same.