I think you're worrying unnecessarily. Unless PTSB have made some specific effort already to get you off your tracker, they are not likely to try now, and most likely they would not be entitled to try. The drop in value of all property has so far not resulted in any reports of banks looking to change the terms due to an altered LTV position. Both journalists and experts (indeed on this forum) have looked for anyone who's encountered such a situation, and so far, I haven't seen any of them come back with stories of people experiencing this.
The letter of approval you got is a relevant document, as it sets expectations, but the real document to read for the final word on this is your mortgage contract. It'll not be easy to read, especially if you don't feel financially savy as you say.
But for what it's worth, I think you'll be fine! Good luck with the completion, and enjoy what sounds like a beautiful new home.