“Croí leoin” is grammatically correct, but if you mean “Lionheart”, rather than “heart of a lion”, you should opt for “Leonchroí”. This is also how it’s rendered on the terminology web site Focal.ie.

As for idiomatic usage, there is a song by Clannad titled “Croí cróga” (“Brave Heart”), but in general, Irish doesn’t seem to use such collocations. It would be more natural to use the adjective on its own, e.g. “gaiscíoch cróga” (“brave warrior”), “fear cróga” (“brave man”), “bean chróga” (“brave woman”.

Expressions featuring the word “croí” (“heart”) tend to refer to feelings, rather than braveness: “cuireann sé gliondar ar mo chroí” (“it delights me”), “bhí a chroí briste” (“his heart was broken”), “crá chroí” (“torment”, literally “torment of the heart”), “cara mo chroí” (“my bosom friend”), “d’ardaigh sé mo chroí” (“it lifted my heart”), “ní raibh a chroí ann” (“his heart wasn’t in it”) etc.

Athbliain faoi mhaise dhaoibh uilig! 🙂