Permanent TSB (PTSB) says it has entered a Memorandum of Understanding to acquire €7.6bn of assets from Ulster Bank.
Ulster Bank announced in February that it was withdrawing from the Republic of Ireland after more than 160 years.
The bank said at the time it was in talks with both AIB and PTSB over the sale of portions of its Irish loan book.
This PTSB deal would include 25 branches, as well as a portfolio of performing non-tracker mortgages.
The bank would also acquire business loans and the Lombard Asset Finance loan business.
As part of the deal Ulster Bank's parent company, Britain's NatWest Group, would become a shareholder in PTSB.
PTSB chief executive Eamonn Crowley told Newstalk Breakfast they will be taking on some 500 Ulster Bank staff.
"This is a very exciting and important day for Permanent TSB.
"But it's more than just a portfolio of €7.6bn in assets, which includes mortgages, a business direct book and also the asset finance business.
"We're also taking on 25 branches, which compliments our national coverage and compliments our existing 76 branch network.
"And indeed we're looking forward to welcoming up to 500 new colleagues from Ulster Bank when we close this transaction."
Asked how PTSB would be financing the sale, he explains: "NatWest will take a 20% share in the bank, so they'll become a significant shareholder from that point of view.
"And we'll also pay some cash as part of the transaction".
He says negotiations will begin before the end of the year.
"At the moment it's a Memorandum of Understanding, we have to get into the actual legal and binding negotiations - which will take place before the end of this year.
"We don't envisage that we will be asking all our shareholders, including the State, to support us in this respect: we believe we will be able to finance it from our own means."
Asked why PTSB bought 25 branches, he says this makes sense for them.
"We are a community bank, we are focused on the communities which we serve.
"But we see the importance of branches to the local communities".
PTSB says Ulster Bank customers, whose loans are intended to transfer as part of this deal, do not need to take any action now.
The bank says it will offer all personal Ulster Bank customers an easy-to-use process to apply for PTSB accounts.
The bank says: "The parties will work together over the next number of months with a view to entering into legally binding agreements in Quarter 4, 2021.
"Subject to legally binding agreements being put in place and subject to receiving all required regulatory approvals, it is envisaged that the transfer of the business will take place within the next 12 to 18 months."