Gráinne na Mara does sound poetic. And I think it would work well since the word for mermaid includes "mhara." The story gives some nuances that she might be more than merely mortal (ehem, lots of M's there).

GRMA, a Bhríd!

GRMA!

After discussing my original suggestion with a native speaker from Connemara, I am resubmitting the following:

A: Mo ghrá thú
B: Agus is thusa mo ghrá-sa

But there are lots of ways to say "I love you" in Irish and as many ways to reply.

Other possibilities:

A: Grá mo chroí thú
B: Agus is thusa grá mo chroí-se

A: Is tú mo ghrá gheal
B: Is thusa mo ghrá gheal-sa

Or I think you can mix and match:

A: Mo ghrá thú
B: Grá mo chroí thú

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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).

As another native I can agree to that.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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GRMA! You have both been a great help with all my questions!