Permanent TSB has confirmed the sale of 10,700 non-performing mortgages.
The portfolio's being bought for €1.3 billion by the Irish-regulated Start Mortgages - an affiliate of US firm Lone Star, a so-called vulture fund.
It includes 3,300 buy-to-let properties, and 7,400 owner-occupier homes.
The average time the loans have been in arrears is three and a half years.
The CEO of PTSB Jeremy Masding says many of the mortgage holders involved haven't engaged with the bank.
He explained: "We will shortly be writing to account holders whose loans are in the portfolio to inform them that their loans are being transferred.
"Very importantly, [we will] remind them that in Ireland, when a loan is sold from one institution to another, the protections which are in place with the selling institution travel with the loan to the new owners."
Fianna Fáil accused Permanent TSB of 'taking the easy way out' with the portfolio sale.
Michael McGrath, the party's finance spokesperson, said: "While it is a positive at least that Start Mortgages is a regulated entity, there will be understandable concern among affected borrowers given its background as a sub-prime lender and the fact that it is owned by giant US vulture fund Lone Star."
Taoiseach Leo Varadkar, meanwhile, said legislation will soon come before the Dáil to ensure stricter regulation of vulture funds.