Hi,
can you give the date of purchase and purchase price?
My understanding is that the property may be indexed to account for inflation as it was purchased before 2004.
Leaving indexation aside as you have owned the property for 18 years of which it was your PPR for 13 you are only liable for CGT on a pro rata basis.
e.g. you have a CGT liability of €10,000 if it was just an investment property. As it was your PPR for 13 of the 18 years you will only have to pay €2,778 (€10,000*(5/18)).
You also have an annual CGT exemption on the first €1,270 which can be used if you have not offset it against any other gains this year.