Why do you need a trust at all?
Price of property: €300k
Deposit €30k
Vendor's risk: €60k
Lender: AIB
Why not issue a mortgage of €270k and put €60k in a back-to-back account with AIB.
Repayments are calculated on €270k mortgage.
But, interest is charged on net amount i.e. €260k.
If prices have not fallen by year 5, the bank pays the €60k to the vendor and the mortgage is increased by €60k
Advantages
No risk to borrower or vendor from default of trustee.
No trustee needed so no admin cost
No taxation of interest in trustee deposit account.
Borrower is making repayments based on €270k, so by Year 5, they will have paid off a good chunk of capital.
From the point of view of the builder
They have minimal admin hassle.
As there will be a deferred payment, they can charge a bit more for the property.
If AIB goes bust, then the purchaser pays the €60k to the developer. (Might be difficult to collect, but the deed of sale can give the developer an interest in the property)