Thank you Tranona, good thing I wasn't planning to stay in Spain for longer than 18 months.... and I'm definitely not going to Greece!It is not the registration of the boat, nor your citizenship that is important. For the boat, if it is "UK VAT paid" -that is it was in the UK on 31/12/20 rather than in the EU, and you are UK resident then you can keep the boat in the EU for up to 18 months under TA as explained earlier. Then you have to take it out and re-enter. The 18 months starts when you clear into the EU. Your rights to stay are governed by the Schengen rules which are nothing to do with the boat. However as I pointed out earlier some customs locally may have difficulty with this distinction, although as Irish Rover says he has been doing similar to you from Turkey to Greece without problems.
This type of situation is one of the complications arising from Brexit, made more complex by the overlap between the Common Travel Area and Schengen which allows Irish citizens to have virtually equal rights in terms of freedom of movement in both the UK and EU, but if resident in the UK their boat is "third country" from a VAT point of view.