Joint mortgage/ deeds with retired Father possible?

No way is a 72 year old going to get a mortgage.

The deal here is that Dad will buy a two-thirds interest in the property for €500k, financed with the proceeds of the sale of his current gaff, and the OP (or OP and his partner together, but from here on I'm just going to say "OP") will buy a one-third interest for €250k, financed with €50k that he already has down the back of the sofa plus a €200k loan.

Note that in this arrangement Dad does not need to borrow anything; he'll get all the cash he needs from selling his existing house. The loan is being made to the OP.

But the loan will have to be secured. OP's thinking is that this will be done by granting a mortgage over the property being acquired. As the property will be jointly owned by Dad and the OP, the mortgage will have to be granted by both of them.

Banks are leery of granting a loan to A, secured by a mortgage over a property owned by A and B. It's perfectly legal, but it puts the bank in a very awkward position if A defaults on the loan; if they enforce their security, B loses his property, even though B hasn't done anything wrong. That's embarrassing to the Bank. It's even more embarrassing where B is (by then) an extremely elderly gentleman. So I would expect the bank to suck its teeth very hard when this proposal is put to it. Not to say that they definitely won't do it, but they mightn't be keen.

A possible varation occurs to me: Does OP have enough equity in the house he already owns (and that he and Dad will be moving into) to offer that property as security for the loan he needs? I think a bank might find that an easier proposal to accept.

Also need to think about estate planning with regard to the new property. Assuming there's just OP and his brother, when Dad dies is the expectation that his estate will be divided equally, and OP and his brother will jointly inherit Dad's share in the new property? That would leave OP with a two-thirds share (one-third that he bought plus one-third inherited from Dad) and brother with a one-third share, inherited from Dad. Is brother in the US going be happy owning a one-third share of a rental property in Ireland? If not, could OP buy out his one-third share? Or would the property have to be sold at that point? Best to talk this through and make sure there's a shared understandign with everyon affected before going ahead.

(If Dad's estate consists largely or entirely of his two-thirds share in the new property then, based on the current value of the new property, there will be no CAT liablity for OP or his brother on Dad's death. But if the new property appreciates signficantly in value and/or Dad has signficant other assets then it is possible that the parent/child threshold will be exceeded and there will be some CAT liablity.)

One other factor: Dad's current house is his PPR and any gain on disposal is CGT-free. However if this plan goes ahead the new property will not be the PPR of either Dad or OP, and gains accruing in it are going to be taxable. It doesn't matter if Dad holds his interest in the property until he dies, but if he disposes of it while still alive (e.g. to finance a care home, should he need that) then there will be a CGT liability on the growth in value since the new property was acquired.

You might say that. My bank manager told me any BTL has to be paid off by the age of 70 and even then the max BTL allowed is 50 %.

By the time the o/p sell the houses and sources a BTL his father will be likely 73 or 74. Banks are not particularly impressed by a low BTL since they have had so much difficulty realising security on BTLs. There are still BTLs going into BIDx1 arising fromn Celtic Tiger era loans. In the majority of cases the banks sold on the loans at a loss to funds.