My American woman wanted to get a tattoo of a heart on her arm with a phrase meaning "She wears her heart on her sleeve".

It took me two days to talk her out of anything like "Tá a chroí ar a mhuinchille aici"

I think we can get the double-meaning (literal heart-tattoo on her arm + figurative meaning of 'wearing one's heart on one's sleeve') using this one from Ó Dónaill, 1977 – https://www.teanglann.ie/en/fgb/barr- : "Bíonn a ngrá leo ar bharra a ngéag, they wear their hearts on their sleeves."

How would I make that 3rd person feminine singular? I've got bíonn a ngrá lei a bharra a ngéag. Is that correct? Just want to make sure the séimhiú and urú are right. Is it ngéag, géag, or ngéaga?

When the possessive pronoun "a" means "her", it causes neither séimhiú nor urú, so you'd have:
bíonn a grá lei ar bharr[a] a géige [with sleeve in the genitive singular form]
I assume that bharra is a dialectical form, and you could do without the final "a" in the word (which will disappear anyway when the phrase is spoken).

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Could we put croí in where Ó Dónaill had grá, or would that ruin the expression?

Yes, I don't see why not:

Bíonn a croí lei ar bharr[a] a géige

I hadn't even noticed that your dictionary example used "love" instead of "heart"!

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

barra is plural, barr singular

Foclóir has this:

to wear your heart on your sleeve

"when she speaks she wears her heart on her sleeve"

nuair a labhraíonn sí is óna croí amach é

Would "Is óna croí amach é" work?

Or why wouldn't she want to say "I wear my heart on my sleeve"? Is ó mo chroí amach é.

From de Bhaldraithe there's "Don't wear your heart on your sleeve"

ná bíodh rún do chroí ar do bhois agat.

Bíonn rún a croí ar a bois aici. (She wears . . . this is also in Foclóir)

OR "I wear . . ." > Bíonn rún mo chroí ar mo bhois agam.

Tim

Hi Tim,

Those break the pun. It's a heart-tattoo on her sleeve.