Idiomatic phrases don't necessarily translate well between languages. Nonetheless I found the following ways of saying 'Follow your heart' at
http://www.focloir.ie .
Lean mian do chroí (literally: Follow your heart's desire)
Téigh de réir do chroí (literally: Go according to your heart)
Déan an rud atá ar do chroí (literally: Do the thing that is on your heart)
As a variation on the last one above, maybe you could also say the following. (On second thoughts, I'm not sure if it's grammatically correct.)
Déan cad atá ar do chroí (literally: Do what that is on your heart)