AIB chief executive David Duffy on Tuesday told the Oireachtas finance committee that the letters were sent to 5,984 account holders between April and June of this year threatening legal action, including repossession.
Mr Duffy said these customers were typically in arrears for between two-and-a-half to three years and had consistently failed to respond to attempts by the bank to engage with them on their debts.
Bank of Ireland chief executive Richie Boucher said the bank was offering deals to customers that it was confident would help to keep them in their homes.
Mr Boucher said the number of Bank of Ireland customers in early arrears fell in "June, July and August".
He told the committee that 11,774 mortgages were in default at the end of March and that by the end of June some 3,103 of those were in a legal or resolution process and 3,164 had been restructured or cured.
At 53pc of cases, this was far above the Central Bank's target of finding a sustainable solution in 20pc of cases.
Ulster Bank chief executive Jim Brown has said that 35% of the bank's customers who are in arrears are not engaging with the bank or are not making any payments.
Some 2,000 Permanent TSB mortgage holders are selling their property as part of a the bank's solution to arrears this year, the Oireachtas finance committee heard today.
The bank's "assisted voluntary sales" or closure solution meant the bank worked with the borrower to sell the property. This figure "doubled" to "roughly 2,000 from June to September, Shane O'Sullivan, head of the bank's asset management unit said.
Permanent TSB has "offered" solutions to 26% of mortgage holders in arrears, ahead of a target of 20pc set by the Central Bank.
Sinn Fein Finance Spokesperson Pearse Doherty expressed shock at the revelation that some 14,721 repossession letters have been sent by the four banks that appeared before the Oireachtas Finance Committee this week.
Deputy Doherty said an additional 2,439 requests for voluntary surrender were sent out.
"This week's revelations in the Finance Committee from Bank of Ireland, AIB, Ulster Bank and PTSB that they have sent out 14,721 repossession letters and 2,439 requests for voluntary surrender are shocking. This government caved in to the bank's demands by reversing the Dunne Judgment in their favour and changing the Code of Conduct on Mortgage Arrears to allow them to apply greater pressure to struggling homeowners."
"The most galling thing about the banks' appearances this week is that they are including sending letters threatening repossession as 'solutions'. The Central Bank set at a target of 20pc of offers to be made by the banks to mortgage holders in arrears of over 90 days. The banks are claiming to have met this target but only by sending out repossession letters."