There should be no tax imlications at your end.
If you purchase the site from your parents for €100,000 and it is worth €100,000 then it won't be considered a gift. Because although if it was a gift it would be under the CAT threshold, it would carry forward and be included in any future inheritance you might receive from your parents when your inheritance tax is being calcualted.
Also, stamp duty is payable on the transfer of property regardless of whether money changes hands or not. For non-residential property the rate for €100,000 is 6%, however I have a feeling this is for commercial property and not sites.
Perhaps someone could clarify what the stamp duty rate applys to sites?
Also, if stamp duty is payable, because it's from your parents it will be half the normal rate.
Finally, your parents are likely to have Capital Gains Tax implications if the dispose of the site - again regardless of whether the yreceive money for it or not.