How would you order Guinness in Irish? My guess would be

Ba mhaith liom gloine Guinness, le do thoil

How close am I?

That's correct

Or even just:

Pionta, le do thoil. "A pint, please."

(You'll be given whatever brand of stout the pub stocks.)

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[hr]Múinteoir Gaeilge - Irish Teacher[/hr]
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).