They would be subject to Gift Tax (CAT) on the value of the rent less the €3,000 annual exemption. However, as a parent can give over €310,000 in lifetime gifts, this should not be an immediate issue.
If you sell the property at some later stage, you would pay CGT on the profit you make.
Being a landlord is just too much hassle these days. I am not sure if there is an exemption for houses let to family members. But if there isn't, you would only be able to increase the rent to a new tenant after they leave by 4% per year.
What you propose is just too much hassle unless it's a very short-term solution.
I think it would make more sense to give them a formal mortgage for the property and let them buy it themselves. That would mean that they would look after it better. They would repay the loan to you which would be a good discipline for them. And any increase in value would be exempt from CGT as it would be their family home.
The interest which you should charge would be subject to Gift Tax, but as before, this would not be an immediate problem.
The downside to this is that you would be tying up your cash. If they don't repay the loan, there isn't very much you could do about it.
If one of them decides that they want to move on and buy a house of their own, having a mortgage, even from their parents, could be a problem. So maybe you could do two separate mortgages so that one person could sell their share of the house (to you?) and repay the mortgage. Take legal advice on this. A normal mortgage might work also.
If you go down the route of joint purchase of the house, then you should make sure that they do an agreement between themselves on what happens if one wants to sell. It would appear to me that the correct solution would be for them to be obliged to sell to you at the market price as determined by a valuer.
Brendan