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The 3 (or 4...) Rphonemes of Gaelic (Irish/Scottish/Manx)

The first (.i.e, the 4th...) R is theoretical and is no longer phonemic (.i.e., considered as capable of changing a word's meaning when pronounced);

Strong Slender R [R'] = a palatal trill (a rolled R with the tung's middle positioned to say yee, like Czech r^)
as in Old (and Modern) Irish/Gaelic
rí /R'i:/ 'king',
Béirre /b'e:R'E/(has regularly become modern Béarra /B'e:Rə/ 'Bearra (Peninsula),
eirr /eR'/ (prepositionalcase of earr '(tail)end, extremity'

even in the early Old Irish/Gaelic Era c.600900, Strong Slender R /R'/ merged into Strong Broad R

Strong Broad R = [R] = a velar/nonpalatal trill (a rolled R like stereotypical Scottish R, Spanish r and rr), as in;
roth /Roh/ 'a wheel'
carraig /KaRəG'/ 'a rock'
barr (Old Gaelic /BaR/, Modern Gaelic /Ba:R/ as if bár(r)) 'a top; a crop'
Strong Broad [R] has become Weak Broad [r] in;
Munster, without exception
generally in Counties Galway (Cona Mara) & Mayo, (especially among middleaged and universally(?) among young speakers), but it seems to have survived in East Connachta (Co. Roscommon, and? Co. Sligo) see Heinrich Wagner's Linguistic Atlas & Survey of Irish Dialects, volume 1 (linguistic maps) and volume 3 (phonetictranscriptions from Connachta)
less generally in County Donegal in Ulster.

Weak Broad r = [ɾ] = a velar/nonpalatal tap (like an extremely short D)
as in
a roth /ə ɾoh/ 'his wheel',
cara /Kaɾə/ 'friend',
ór /o:ɾ/ 'gold'

Weak Slender r = [ɾ'] = a palatal tap (like an extremely short D, with the tung's middle positioned to pronounce yee)
A rí! 'O king! (slender r at the start of words is practically always broad r nowadays, so that a rí! is pronounced with broad r /ɾi:/)
náire /Na:ɾ'I/ 'shame'
b'fhéidir /B'e:D'iɾ'/ ''It would be possible')