Buy-to-let investors are often attracted to the tax-efficient merits of interest-only mortgages.

Interest on borrowed money "employed in the purchase, improvement or repair" of property is an allowable expense that can be offset against rental income.

"If the mortgage is reducing each month, as with an annuity mortgage, this expense is also reducing and hence the tax liability is increasing," explains Ms Sarah Wellband, associate director of mortgage advisers REA.

Under interest-only mortgages, repayments consist entirely of the interest due with no dent being made in the capital balance until the end of the loan or until the end of the maximum period on which the lender will allow only the interest to be repaid.

Then, and this is the potentially sticky part, it must all be repaid at once.

Permanent TSB and IIB offer investors up to 90 per cent finance, with most other lenders lending 80 to 85 per cent.

However, EBS says 100 per cent finance is available where additional property can be offered as security so that the overall loan-to-value ratio is 75 per cent or less, and most lenders will adopt a similar approach when calculating a loan offer.

Is it really a good idea to postpone the pain of paying off such high mortgages?

"The danger of interest-only is that in theory you could get to the end of the term and have no means, other than by selling the property, of repaying the capital," Ms Wellband says.

"Given that most institutions only allow interest-only on investment properties and then only for a maximum of 10 years, this is an unlikely scenario," she adds.

Permanent TSB has a three-year interest-only repayment option, while EBS, First Active and IIB give interest-only over five years.

Bank of Ireland, meanwhile, will allow buy-to-let investors to spend up to 10 years repaying only the interest.

Most other lenders will consider interest-only on a case-by-case basis, depending on the loan-to-value ratio and borrowers' income.

However, most lenders will do it for the length of the loan, according to Mr Bastable.

"Even those who say they will do it for three or five years will review it. They don't want to lose the business."

Interest-only mortgages are very popular with investors, confirms Mr Bastable, and can solve a dilemma for those trying to plan ahead during the same years as those when demands on income are at their most intense.

"At 35, a couple might have children who are three or four years or five or six years of age. They have 12 or 15 years to go of very focused spending. These are the critical years for disposable income, no matter if you are a low-income or high-income earner. Yet you should invest," he says.

But if the couple pays interest only for 10 of those years, it will ease their monthly burden. Then, they can start knocking substantial chunks off the capital sum.

Alternatively, if they have the means and the value of the property has appreciated, they can use the leverage gained to borrow again.

Spend too long on the interest-only portion and investors may have to sell in order to repay the capital - something they might not want to do if the intention was to keep the property in the family.

But Ms Wellband says interest-only is tax-friendly for investors in the short term regardless of the property's eventual use.

"Once the property is no longer being let the mortgage should be switched to annuity. If the children intend to buy the house from the parents, it will then become their main residence and the mortgage will qualify for owner-occupier tax relief," she says.

Interest-only mortgages may also be advantageous for other borrowers, such as people relocating to Ireland for short periods of time, Ms Wellband notes.

People who are working here for, say, five years and who do not want to rent may buy a property they are intending to sell when their contract ends: interest-only is a way to keep repayments down in the meantime.

Sometimes it suits individuals - such as the self-employed and those whose income consists of a low basic salary plus large bonuses - to make the minimum monthly repayment possible and then make lump sum repayments as and when they wish, Ms Wellband says.