"I do not shine."
How is it not idiomatic? The literal translation doesn't really make sense, you have to learn it's meaning separately from the words (save for the fact that there is a similar idiom in English - to take a shine to something.)
Even if it is used heavily, an idiom is an idiom. Most idioms are used a lot, in fact.
Dia duit isn't an idiom, it is simply a different phrase altogether - one that is straightforward. A thing doesn't literally shine with you, and saying that makes no sense without learning that is means to like/enjoy something. With dia duit, however, you are saying exactly what is meant: that I wish god be with you, to protect/guide/whatever.
Most people don't attach that meaning to it anymore, it seems, just as with goodbye. That doesn't make it an idiom.
I would say an idiom is a phrase that, even if you know the definition of every word it contains, you can't make sense of it without it being explained to you — you can know "he", "kicked" and "bucket" but will never work out it means someone died until your told.
I'm just a learner but "taithin" doesn't feel like this to me as there is no "more literal" way to say "I enjoy something" — it's not colourful language, but rather another literal definition of the word.
I found 'taitin' on teanglann https://www.teanglann.ie/en/gram/taitin
I've also been using ctrl+f on this page to help me find the root word so I can practise conjugations, though I'm not sure if they are all correct (and taitin is actually not even on this list), but using it with teanglann's grammar section has been really helpful for me. https://en.wikibooks.org/wiki/Irish/Vocabulary/Verbs
The purpose of this exercise isn't actually to teach people how to say "I don't shine", it's to help people to get used to the difference between Ní thaitním and Ní thaitníonn sé liom.
taitin is not the Irish for "like" or "enjoy", even though it is used in the translation of sentences that contain "like" or "enjoy".
Ah ok, I see what you mean. The word "shine" threw me off because I don't associate it with "like" or "enjoy" and I'm not sure anyone else does either. Maybe "shine" in the question should be replaced with "like/enjoy" because other people seem to have been confused by it too. Go raibh maith 'ad, a Sliotair.
The whole point of the exercise is to remind people that the verb taitin doesn't mean "like" or "enjoy". People get it in their heads that you use taitníonn to say "like", and think that they can say "taitním an bia" for "I like the food", or "thaitin mé an cheolchoirm aréir" for "I enjoyed the concert last night".
This exercise doesn't need to be changed, because this exercise is not the source of the confusion - Ní thaitním does not mean "I don't like/enjoy", it means "I don't shine". Ní thaitníonn an bia liom means "I don't like/enjoy the food", but the preposition le is a requirement, and an bia is the subject of the verb, not mé.