"My heart."

[grainnenim]

Also a term of endearment :)

[galaxyrocker]

I feel a chroí would be more natural, similar to a rún though a chroí is used for 'adressing someome you don't know'. It's also like ❤❤❤ you'd day ó a dhia instead of ó mo dhia

[scilling]

The FGB offers a rún mo chroí, so perhaps mo chroí as an endearment elides the initial vocative part, e.g. a rún or a stór.