The Competition and Consumer Protection Commission is to carry out a full "Phase 2" investigation into the proposed deal by AIB to buy a portfolio of performing commercial loans from Ulster Bank.
The Commission said that after an extended preliminary investigation, it has determined that a full investigation is required in order to see if the proposed transaction could lead to a substantial lessening of competition in the State.
In June, AIB announced the proposed deal to buy about €4.2 billion of performing corporate and commercial loans from Ulster Bank.
AIB said the deal was reached further to the Memorandum of Understanding that was announced in February when Ulster Bank owner NatWest said it was withdrawing from the Irish market.
Under the terms of the proposed deal, about 280 Ulster Bank workers who are directly involved in the servicing of the loan book would transfer to AIB.
The CCPC said that any parties who would like to send in submissions are invited to do so, via email to email@example.com, by 4.30pm on Friday January 21, 2022.