The Irish Bank Officials Association has estimated the cost to individual staff members at between €2,400 and €5,900 each.
Under a Strategic Transformation Programme negotiated in 2005, staff were to receive up to 6% of salary in free shares under a gain-sharing arrangement.
However, that was conditional on Bank of Ireland meeting financial targets relating to earnings per share growth and cost management.
Last week, the bank announced that while its cost saving targets had been met, earnings per share targets had not.
In light of that, the bank considered that a 3% share bonus would be fair and equitable in the interests of all stakeholders - including employees and investors.
The bonus scheme will cost the bank €20m.
The IBOA described the move as a serious breach of trust which would de-motivate staff.
General Secretary Larry Broderick said staff had met their side of the bargain, delivering 2,100 job cuts and savings worth €147m a year.
It criticised the bank for making a public announcement before referring the matter to arbitrator Kieran Mulvey.
After meeting both sides last week, Mr Mulvey said he was unable to make a recommendation because the two sides are now 'polarised'.
However, he remains available to assist the parties if required, he added.
