Also a term of endearment :)
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 hoe you'd day ó a dhia instead of ó mo dhia
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.