Are these completely interchangeable or do people just mix them up a lot?

They both seem to mean 'available', but I thought there was a difference, but I can't think what it might be now.

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I think they are mostly interchangeable, except that ar fáil may have a slight sense of being "made available" by someone rud a chur ar fáil, whereas I don't think you can say rud a chur le fáil*.

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Múinteoir Gaeilge - Irish Teacher
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).