Thanks for your inputs.
It is a brother and sister (both basically computer illiterate hence I'm asking the question). He lives in Ireland, she in the UK.
I told him that Irish tax laws on gifts to siblings are pretty hefty- my understanding it is something in the region of 33.5k, accumulative, tax free.
Thus, he wondered if they bought a house together, if he died first, what kind of inheritance tax would the sister have to pay and should she die first what would he have to pay tax wise.
As far as I understand it, he would be the main contributor to the purchase as he would be living in the property, probably 75/25 split. He also wants to know could they have a 50/50 share in the property even if he was paying more towards the purchase. Maximum purchase price would be in the region of 135k.
A though has just struck me as I write, he could probably gift his sister an amount to make it a 50/50 share. (pause while I check UK taxes). But just checking the UK system, he may gift £3,000 per year or £6k in the first year.
From my knowledge of the family, he has cancer so is likely to die first.