Would this actually be said? I feel like this would always be said "is tusa" but maybe that's just me!!!

You might have
Is tusa stór mo chroí
...that would do it for me if someone whispered it in my ear...
... sets me thinking of the Irish for 'slim chance'... LOL

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Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

I used to think that too, but I have seen is tú ~ in many old songs since then.

There are three viable constructions with the same meaning:

Is tú (mo stór).
Is tusa (mo stór).
(Mo stór) thú.

Any term of endearment can be substituted into these in place of mo stór.

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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).

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