Hi, contacted Revenue it appears that your parents would not be entitled to Group a thresehold (€400,000 plus limit)
however they may avail of the dwelling house exemption if they satisfy certain conditions which are:
1) have continously occupied the dwelling house as there main residence for a period of three years prior to the gift
2) continues to occupy that dwelling house as there main residence for a period of six years commencing on the date of the gift
then that gift is EXEMPT from CAT. Now there are a couple of other small conditions such as your parents must not be beneficially entitled to any other dwelling house at the date of the gift.
sorry for the confusion, hope that clears it up.