VAT query when buying a motorbike up North....

Im looking at buying a demo bike from up north, 6-7 months old, currently with approx 1600 miles on it.
I realise the low milage (<6000kms) means its liable for VAT, even tho its older tha 6 months (still classed as new vehicle).
The dealer is willing to sell it to me with the UK VAT deducted, as its a demo bike it has not already been paid.
So I then pay the VAT in the Rep. of Ireland.
My query is what figure is the VAT I pay calculated on?
Is it the price I pay up North (converted to € at an agreed/official rate), or is the price I pay up North(converted to € etc) + VRT?
If its the latter, the VAT would obviously be much higher.

Appreciate any help on this, as its hard to get clarity on the matter.

The Vat is calculated on the invoice price.The VRT is calculated using the ccs of the engine @ €2 per cc up to 350 ccs and €1 per cc thereafter.This is then depreciated by a set formula based on the bikes age.