Putting myself out there, I'm an American with an Irish girlfriend who will be celebrating a 6 months anniversary. I want to surprise her with a short phrase and need help with the translation and pronunciation. I would be very thankful for any help I can get!

"The last six months have been incredible and I'm happier when we're together. I'm so thankful that you crossed into my life. Love."

Bhí siad dochreidte na sé mhí seo caite
The last six months have been incredible

agus bím níos sonasaí agus sinne le chéile
and I'm happier when we're together.

Táim chomh buíoch sin gur thrasnaígh ár saolta a chéile
I'm so thankful that our lives crossed [a bit of paraphrasing here]

Le grá
Love

Corrected

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

Thank you so much! I really appreciate it. Can I also get help on pronunciation? I'm clueless!

I suggest waiting for some comment on what I gave you as a translation, because someone may catch errors or have a better idea.

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

A Chaoimhín, you have a kind of reverse TSF error going there. Dochreidte is an adjective and so requires tá/bhí rather than the copula is/ba. For the emphatic use of the copula, parallel to Is maith é sin, etc., iad would not be not correct.

Bhí na sé mhí seo caite dochreidte ...

I think, however, in this case "incredible" is not really "unbelievable", but just a superlative like "fantastic" or "terrific".

Similarly, "so" is used in American English to mean "very".

Here's a second pass at it:

Bhí na sé mhí seo caite thar a bheith iontach

agus bím níos sonasaí nuair a bhíonn muid le chéile.

Táim an-bhuíoch gur chas ár saolta ar a chéile.

Le grá.

More suggestions or corrections welcome ...

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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).