What should someone who has made millions from BTC do?

that is my understanding of the criteria for becoming tax resident in another jurisdiction. Am I wrong or am I missing something?
Assuming you are Irish and have been here for a long time, you will not lose your status as ordinarily resident for tax purposes, for 3+ years after you leave the State.

So an Irish-domiciled individual will be liable for CGT on worldwide gains during the year of leaving Ireland and for the following three tax years.

You may have a measure of relief under a DTA, but if the whole purpose of the exercise was to go somewhere that doesn't tax the gain, then DTA relief is irrelevant.