What about
Ní mheathann an (cineál) grá is fíre (choíche)

Lots of fine suggestions above.


... and I think I'd put the é after choíche, though that is probably optional.

On a slightly different tack, perhaps:

Is buan an fíor-ghrá. "True love is enduring/permanent."

buan can be used for "permanency" with regard to colours, memories, and love.

And to bring in some of the parenthesis of the original, perhaps:

Is buan an grá, an grá is fíre.

Await further input ...

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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).