Irish CGT will be based on:
Sale price (converted to € at appropriate rate)
- costs on sale (converted to €)
= net proceeds
Minus the aggregate of
Purchase price (converted to €)
+ Costs on purchase (incl. Stamp duty)(converted to € at appropriate rate)
+ Enhancements expenditure(you guessed it, converted to € at an appropriate rate)(eg. the cost of an extension, or similar capital improvements).
That will give you your Irish chargeable gain.
If you have any CGT losses they'll be deducted from your gain, and you have a 1,270 annual allowance.
After all that you'll have the amount you are taxed on, at 33%. Then you can get a credit for your UK CGT paid, against this tax.
When I say to convert amounts to Euro at an appropriate rate I mean the rate at the time. Depending on relative exchange rates at the relevant times, you might find your € gain is increased or decreased.