The Irish Mortgage Corporation says the number of first time buyers taking loans over a 40-year term has more than doubled in the last 12 months.
It says that almost a fifth of all first time purchasers it deals with are now opting to repay loans over four decades.
This means buyers have lower mortgage repayments each month than had they borrowed over a shorter term, but IMC Director Frank Conway says most people will review the situation a few years into the life of the mortgage.
He says the number of customers taking out these long-term loans has increased from 2.7% three years ago to 18.2% in the last six months of 2007.
IMC estimates that total interest payable on a €300,000 mortgage over 40 years is €114,000 more that the amount payable on the same loan over 30 years.
It says most buyers that opt for such a long-term mortgage do so to keep repayments down, before reviewing the situation and usually refinancing to a shorter term.