The Irish Postmaster's Union has said 200 post offices face closure in the next 12 to 18 months if the Government does not continue with the transformation payments the outlets currently receive.
The group told the Joint Oireachtas Committee on Transport and Communications Networks that more postmasters will continue to retire as their operations become unviable.
It said some postmasters will see a €10,000 drop in income as a result of the cessation of payments in June.
IPU General Secretary Ned O'Hara said: "The day of reckoning has arrived for the Post Office Network and it is now about actions and not platitudes.
"The Government needs to act quickly and decisively to keep post offices open by ensuring a financial intervention is put in place before the end of June this year."
Senator Timmy Dooley of Fianna Fáil said: "The real crisis is now. That money needs to be in place by June to ensure we don't see the closure of any more posts offices."
He added: "We need to see the haemorrhage stop. I'm disappointed we are into another round of reviews."
Ireland's postmasters are contractors who are paid for the transactions carried out in post offices.
They have raised fears about the impact of the end of transformation fees which are due to expire in June.
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Labour's Duncan Smith said: "I hope we will not be here in a year's time, but with less post offices."
Fine Gael's Joe Carey said that "there is an urgency in this," and called on the committee to make a "strong recommendation so we can stand by the postmasters' union."
Sinn Féin TD Darren O'Rourke said he had seen three post offices close his own constituency and the effect had been profound.
Mr O'Hara said: "People can't continue as it is; either we are required or we are not required."
Last week, Minister for State Hildegarde Naughton decided to establish a new Inter-Departmental Group to examine directing more Government business to the Post Office network.
Earlier, speaking on RTÉ's Morning Ireland, Mr O'Hara said post offices have been particularly relevant throughout the pandemic and a range of services, such as motor tax payments, have been identified as one which post offices could provide.
Mr O'Hara added that post offices carry out 30 million ESB transactions and serve 1.3 million people. Compared to the money being put into banks, he said, the IPU is looking for "peanuts".
Mr O'Hara said that a committee, set up to establish ways of directing more business to the post office network, is not due to report until the end of July, and his members are making decisions about their futures now and cannot wait that long.