I was told Welsh speakers greet people they don't know bilingually - "Shwmae? Hello" This gives the other party the choice of answering in Welsh if they can, or in English if they can't speak Welsh.
I think more Irish people (and learners like us) should adopt this practice: "Dia dhuit, Hello!" It would let other Irish speakers know you are interested and them a chance to open up a conversation.
I broke through the stranger barrier on the Aran Islands for the first time by using "Go rabh ma' 'ad" instead of standardised pronunciation.
I've also had a few audience members come up and speak Irish to me after singing songs in Irish in our band. (One of them was an interviewer for Radio na Gaeltachta in Japan covering the World Cup Soccer - man was I _not_ prepared for that.
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But my point is, people won't know whether you do or don't speak Irish if you don't show them first (the Fáinne Óir is another way).
As for the Gaeltacht, I am sure there must be regional meetings (particularly concerning the Gaeltacht itself, for instance) where people know that most people there will be able to speak Irish but don't necessarily know all of them personally. I imagine in situations like that the formal greeting would be used, wouldn't it?