Could someone please tell me the best way to say an Irish speaker/learner from the United States? What would be the plural?

The best I can do with the internet has been "gaeilgoirí Meiricaénacha" Is that total nonsense? (Most likely!)

Thank you!

I think I'd say:

Gaeilgeoirí Mheireacá (GC) "Irish-speakers of America".

In "standard" Irish, that would be:

Gaeilgeoirí Mheiriceá (CO) "Irish-speakers of America".

Await further input on that ...

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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).

Yup. Or simply what the OP was getting at, which would be Gaeilgeoirí Mheiriceánacha I believe.

An Irish speaker from America : Gaeilgeoir as Meiriceá
Irish speakers from America : Gaeilgeoirí as Meiriceá

"Gaeilgeoirí Mheiriceá" is rather "the Irish speakers of America" or the American Irish-speakers

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Thank you very much! I think I'm going to go with "Gaeilgeoirí Mheiriceá." How would that be pronounced? (I have a vague idea, but I'm not used to hearing spoken Irish, so I'd like better opinions...)

Actually, I think Lughaidh has a point, Gaeilgeoirí Mheiriceá implies "all Irish-speaking Americans", not just some.

Conamara-style pronunciation:

Gaeilgeoir as Meiriceá "an American Irish-speaker"
G(w)EYL-g(y)orh uss MEH-rhih-k(y)aw
/ge:l´g´o:r´ a:s m´er´ək´ɑ:/

Gaeilgeoir as Meiriceá "(some) American Irish-speakers"
G(w)EYL-g(y)orhee uss MEH-rhih-k(y)aw
/ge:l´g´o:r´ i: a:s m´er´ək´ɑ:/

Slender r in the middle or at the end of a word in Irish has a buzz somewhat like the French j in je, which I have written as "rh" in the phonics.

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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).

Yes, I agree completely. That's partly why I gave the adjectival version.