They have stumped up 20,000 each to open share accounts in the building society, which last week reported a net 1bn increase in deposits during 2005. But deposit returns on most share accounts have not budged in months, even though the European Central Bank has hiked the base rate of interest twice in the last four months.

Much of the carpetbaggers' money has ended up in a seven-day notice account that was heavily promoted by the Irish Nationwide throughout last year.

The account pays 2.35% interest on the minimum 20,000 deposit, the same return that the Nationwide was paying before the ECB began increasing rate last December.

The long-awaited sale of the Nationwide will result in bonanza windfalls for qualifying members, who comprise 100,000 depositors and 20,000 mortgage holders.

Scott Rankin, a bank analyst at Davy Stockbrokers, has estimated that the building society could fetch more than 1.5bn, giving average windfalls of at least 12,500.

Rankin identified Bank of Scotland and Danske Bank, the Danish owner of National Irish Bank, as potential purchasers of the Nationwide. The building society's network of 50 branches would be attractive for any acquirer looking for a bigger slice of the lucrative retail banking market.

Despite the failure to improve deposit rates, the Nationwide's managing director, Michael Fingleton, has pledged to capture a bigger share of the savings market.

"There is a battle being fought in the deposit market as each institution seeks to protect and build up its deposit base, " he said. "You can expect better and more varied rates to be offered to depositors as the competition intensifies.

The Irish Nationwide, with its strong balance sheet and low cost-income ratio, is very well placed to compete strongly on both sides of the balance sheet on service and price, irrespective of the competition."