Let's call it the €150bn question. That's the amount of loans Dublin-based banks have loaned out on home mortgages.

Accounting for almost 60% of all the €260bn in property-related loans on the banks' balance sheets, home loans tower above the €90bn of compromised commercial property loans that the National Asset Management Agency (Nama) will take from five lenders.

But compared with the fuss around the Nama loans, the quality of the lenders' home loans books surprisingly attracts little attention. The banks were slow and disingenuous about owning up to the problems of the commercial loan books ? pretending for a long time that their commercial property loan books were in good shape. Their economic forecasts were misplaced, notably on the one big number that affects the ability of home owners to meet their monthly mortgage payments ? unemployment.

As recently as early spring, AIB senior bosses said jobless rate would peak, at worst, at 13% next year. So the €150bn question is this: how can we be so sure that, despite the assurances, the larger more loan books are in good shape after all?

Colm McCarthy, the chairman of An Bord Snip, said he was concerned at the problems facing the banks even when Nama relieves them of their soured commercial property loans. McCarthy warns that the provisions they have put aside for their now souring residential home loans is too low.

"The most recent numbers were from AIB. I haven't done any huge research on it but they [loan loss provisions] look low to me," he told the Sunday Tribune. "We know how slow they were in admitting to the commercial loan losses. We are in a very deep recession. We know incomes are being squeezed. We also know we are at the bottom of the interest rate cycle. There has to be a significant hit to the balance sheets of the banks far outside the Nama world."

Even in Germany, which is supposedly among the first to emerge from recession, they are fretting about the delayed effects on their banks from rising unemployment next year. And the international rating agencies also warned about the threat of rising negative equity facing householders here.

Last week, ratings agency Moody identified the rising jobless rate as a threat to the Irish banks' home loan books. "The risk of borrowers becoming unemployed continues to increase," said Nitesh Shah, a Moody's economist. "While leading economic indicators do not show any signs of recovery in Ireland, the performance of mortgages may not yet reflect the full depth of the current recession," said Shah.

The ratings agencies sample the quality of underlying prime mortgages when banks bundle up the loans to raise money on the debt markets. Moody's latest survey found that the so-called delinquency rate of underlying loans in Irish prime climbed to 2.3% at the end of June from only 1% a year earlier. It defines delinquency in this case as a non-payment on the loan for over 90 days.

Significantly, those economists who have done the most detailed work on employment shake up sector by sector wonder aloud the most about the secondary effects on the banks' residential loan books. "We should all be thinking of it a bit more" said Dermot O'Leary, chief economist at Goodbody Stockbrokers, referring to the residential loan books.

A journey through the aggregate loan books is straight forward. The €260bn of property loans, broadly, breaks down between €90bn for the Nama commercial property and the €150bn in home loans. Other household debt may be significant for some but, unlike elsewhere, is a relatively small proportion when set aside the mortgage debt.

Nonetheless, Central Bank monthly figures show that personal non-business debt people carry on their personal credit cards in June of €2.89bn, including amounts that will be paid off, was slightly higher than last summer and about €550m more than the boom time level of June 2006. The Central Bank figures detail other categories of household loans, but the picture of mortgage debt accounting for the largest share of the personal indebtedness remains un­changed.

O'Leary, who was the first to forecast that the jobless rate would peak at 17%, and remains the most pessimistic, says there is no slide rule that would help to calculate that for every percentage point rise in unemployment, the banks will have to write off, say, a hundred million euro as people fail to meet their monthly payments. Goodbody's forecasts that AIB and Bank of Ireland between them will have a 2% charge across their loan books. Extending that number across the €150bn all the Dublin mortgage lenders lend, suggests that the lenders will write off €3bn losses in bad mortgage loans.

Nobody knows whether the figure will be greater because, unfortunately, past experience of the property slump in the 1990s in London may not be much of a guide because of significant differences in the legal and banking practices ? and in the severity of the recessions this time.

O'Leary believes there are two specific Irish conditions which may be working to delay the effects ? firstly, the social welfare system that pays the mortgage interest payments of households who are in trouble and have negotiated with their lenders.

The second is what he calls the forbearance of lenders after political pressure to negotiate and refinance troubled borrowers, such as extending capital repayment terms.

"It becomes a banking problem if their forbearance is not sustained," O'Leary warns.

'Middle Class Welfare'

Some refer to it as 'middle class welfare' – the means-tested payments by the Department of Social and Family Affairs to households who have got into trouble paying their monthly mortgage payments because of unemployment.

The Department says that at the end of July, 13,616 people were receiving so-called Mortgage Interest Supplement, up from 8,091 people at the end of 2008 and 4,111 at the end of 2007. The estimated expenditure on mortgage interest supplement for 2009 is expected to be about €40m, or €12.5m more than for 2008.

The Department pays the interest payments only, but because in the early years of a mortgage a borrower is paying off mostly interest, not capital, the payment will normally cover the full amount of the monthly payment to the bank.

Many of those qualifying for the mortgage payments are believed to have taken out first time buyer loans during the boom years and have since lost their jobs.

The department said: "Once the Community Welfare Officer is satisfied that they were originally able to afford the loan but experienced a subsequent change in their circumstances, the person's entitlement to mortgage interest supplement will be examined taking account of the other qualifying conditions of the scheme.

"A review of the administration of the mortgage interest supplement scheme is progressing. The main purpose of the review is to consider how the mortgage interest supplement scheme can best meet its objective of catering for those who require assistance on a short-term basis, where they are unable to meet mortgage interest repayments on their sole place of residence.

"Legislative and operational issues arising in the existing mortgage interest scheme are also being examined."