I'm fairly well acquainted with the distinctions between Scottish, Welsh, and Irish accents in English, but I just wanted to be clear on one thing. Regardless of when an R is pronounced across these languages, am I correct that Scottish and Welsh English speakers tend to use /r/ or /ɾ/, whereas with the exception of some dialects, Irish English speakers use the more RP sounding /ɹ/, albeit much more frequently than in RP? (On that note, if you happen to be Scottish, Welsh, Irish, or of some other Celtic identity, which R do you personally use?)
If this is indeed the case, does anyone know why this occurs? To my knowledge, no Celtic language contains /ɹ/ but all of them contain /r/ and/or /ɾ/. So why would Irish English speakers have adopted this retroflex phoneme when Scottish and Welsh English speakers do it less often, if at all?
Furthermore, why do some non-Celtic regional accents in England, e.g. Newcastle and Yorkshire, use /r/ or /ɾ/, when many others do not? Is it the influence of Celtic languages in close proximity, or is this all completely mixed up?
IPA in responses will be the most helpful unless you can describe phonetic articulation in really clear detail without it.