thanks to everyone for all these very useful input.
to explain further ; family relations are fairly okay. but there is the sticky question of 'compos mentes' ; my mum is 86. i am being very cautious in so much as i dont want to do anything at this stage which could later be 'challenged'. our solicitor has advised that from the perspective of selling the site, there is no issue of 'compos mentes' but when it comes to 'gifting' it, there could be. hence my caution.
indeed i may have put a few carts before the horse but that is with hindsight. i had no idea that sites/property in ireland would appreciate at such an insane rate.
i did indeed ask the architect for a valuation. he put me in touch with an auctioneer 'friend' of his who did value the site. i stressed that i wanted a completely objective valuation. unfortunately, i didnt get that. when i asked the auctioneer about his valuation which seemed well under the mark and which he charged me 150 euro for, he immediatly started to back peddle and say , well, yes, its probably worth 20% more than i quoted. the architect is quite keen (as i am myself) to see the house built so i guess he felt, in spite of my request, that he was helping me by 'explaining' the situation to the auctioneer.
my siblings (and myself) all live outside the country.
this experience made me realise how 'easy' it is to get a valuation from an auctioneer to say whatever you want it to say. since i am , in a sense , the fox in charge of the chicken house, i really want to go the extra mile to make sure that everything i do, and am seen to do, is above board.