The Minister for Finance has been told that the figure of €700 million that has been set aside in unallocated spending for Budget 2020 lacks credibility.
At the Oireachtas Finance Committee, Sinn Féin's finance spokesperson Pearse Doherty said the Christmas bonus, which cost €285m last year, had not been factored into the calculations.
The minister set out in his Summer Economic Statement last week how tax cuts amounting to €600m could be achieved.
"It's not loose change you'll find down the sofa. If you intend to pay it, why not factor it in?
"Christmas happens every year. The Christmas bonus has to come out of that. Why don't you present a figure that has more credibility, which is a figure of just over €400 million?"
The minister said no decision had been made in relation to what was going to happen in December 2020 yet.
Paschal Donohoe said the IFAC criticism that the level of growth in current expenditure was not realistic had been acknowledged.
"I have recognised that in the medium term expenditure outlook, by increasing the rate of current expenditure for years after 2020 from 2% to 3.25% to deal with charge that's been made, that we've underestimated current spending growth."
The minister accused the deputy of conflating the issues.
"The Summer Economic Statement is to say what level of resources are available, not to make decisions into how those resources are made."
The minister said he had a track record over a number of years of paying a Christmas bonus.
He said he expected that in October 2020 there will be a Christmas bonus, but the decision had not yet been taken.
He said he was focusing on decisions he would be making in October 2019, not October 2020.