I'm not a native, so wait for more input on this but since tá mo chroí istigh ionat "my heart is inside you" means "I love you" idiomatically, reversing it seems like trying to say "you love me". Not quite the intention of the English.

The other suggestions are nice but I really like AnBraonach's Grá mo chroí go deo thú "You are the love of my heart for ever" for getting the gist and a close idiom of the original English. It feels like "there is/will be no one else in my heart but you forever".

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

One thing I think would be useful to get from the OP: Is this a romantic sort of love we're talking about, or is this a friend/brother/sister situation? Some of these suggestions would be less appropriate for the latter situation.

Redwolf

That's what I wanted to say: it's not romantic love. My cousin died at age 17 and he was like my not existing brother to me.

I'm sorry to hear about your young cousin.

Perhaps you may like something more simple: 'Beidh tú liom i gcónaí' - 'You will be with me always.' I know that it is not the original wording that you gave, but I think it matches the sentiment without becoming overly complicated. Feel free to dismiss my suggestion outright!

Edited as per Pennyrat's eagle-eyed instruction below!

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Maybe something like the following would work:

(Beidh tú) de shíor im chroí

(Beidh tú) i mo chroí go síoraí


While the síor bit means eternal, I'm not so sure the first one is appropriate for the future i.e. does it have the sense of 'always' more than 'forever'? I have a feeling it can mean both.

A nice simple sentiment!

I like this one too. Or perhaps even: de shíor i mo chroí thú.

Sorry about your cousin, tide!

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Ultach mé agus Gaeilg Uladh a labhraim go measardha maith!

Just a slight typo, a Shaoirse..

gcónaí

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I am only a beginner. Please get confirmation before using any of my suggestions.

"My heart will be within you forever" doesn't really mean how I'd like it.

And tbh, I'd like a "longer" phrase, if possible, because I'd like to have two lines. In the first "I love you. I miss you." and in the second "I will carry you always in my heart.". The reason is that I want the tattoo on my "inner" biceps. I can show you a first try: https://www.dropbox.com/s/0n6sp9z5tvlcfx2/tatoo.jpg

Hmmm...in light of the fact that this isn't romantic love, I'm wondering if we can still use "mo ghrá thú"? That has a bit more passion attached to it than I would like for this application (granted it's used in songs in reference to Christ, but it still has that passionate connotation).

If we CAN get by with "mo ghrá thú," then I think Anbhraonach's "Mo ghrá thú, Cronaím thú" can stand for "I love you, I miss you." But wait for more people to weigh in on that.

Redwolf

Ceartaithe anois!

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