The problem with the internet is that it just keeps copying itself. Maybe one person repeats the same mistake on multiple sites. Or one person makes one mistake and lots of sites copy it. The worst is when there's one mistake that gets copied around and you try to get it corrected on one site, but someone comes back later and "corrects" it because he sees the other (wrong) form on so many sites....

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A language belongs to its native speakers, and when you speak it, you are a guest in their homes.
If you are not a good guest, you have no right to complain about receiving poor hospitality.

I just tried using two different automated translation tools online, and they both translated "Is í Christine cuisle mo chroí" as "Christine is in my heart pulse"....the phrase "Is Christine an chuisle mo chroí" was translated as "Christine is the pulse of my heart", which is what I actually want to say.

Can someone review this and let me know the correct phrase to use ASAP? I'm scheduled to have this tattoo done TOMORROW, and I thought that I had the translation nailed correctly months ago, but now I'm not sure anymore...

The last version you found is simply incorrect. In Irish, the definite article "an" ("the") would not be needed or used in that sentence, because it is implied in the genitive form cuisle mo chroí. For what you want, Is í Christine cuisle mo chroí is correct, although you can also abbreviate the first word(s) in either of these ways:
's í Christine cuisle mo chroí [that's an apostrophe before the initial "s"]
sí Christine cuisle mo chroí [this is perhaps more colloquial, but is how most people would say it]

Beware of online translators. They can be adequate for getting the gist of something when going from another language into your language, and for translation of frequently used phrases (because they have a lot of hits for them), but they are very poor at fully correct translations, often making mistakes with nuances, idioms, grammar, and syntax. They are not actually translating for you, by the way, but are instead making guesses at how to say things, based on algorithms they use and phrases they find online.

As you were already warned, also beware of IGTF. Many of us on this forum are refugees from that site after it was taken over by a commercial service and went rapidly downhill. The posts on that forum in the last few years are often flawed, although some people on here do go back from time to time to help correct things.

Note that I'm locking this thread now, because you have two threads going on the same topic, and that causes confusion. Please stick to one and add further questions in new posts to it as needed.

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