If you take out a loan today from EBS at 4% to pay off a Buy to Let loan at 6%, then it's clearly a refinancing of a loan and it continues to be tax deductible.
So if you buy a house with €200k cash and later remortgage it, then there is no issue.
However, you have €200k cash.
You are going to pay €100k off your buy to let mortgage
You are going to borrow €100k to buy the house
In your head, that is a refinance, but it's not on paper, so I think Revenue would disallow it.
To be clear, you should buy the house for cash first and then remortgage it.
Brendan