The Communication Workers Union says An Post's decision to shut down postal services is "ludicrous" and unneccessary.
Services are still suspended nationwide following the strike action by 36 workers at IO Systems over new roster arrangements in response to budget cuts.
Sean McDonagh, the National Officer for the Communication Workers Union, says An Post shut down services in a "panic attack" in an effort to force the workers to back down.
He said: "40% of that business could be running this morning.
"We are calling on An Post management to reverse that decision.
"That's absolutely ludicrous what they are after deciding there and it seems to me to be a decision by An Post management that we must force through the change that IO Systems want."
However, IO Systems denies that with their Managing Director, Fergal Lynam, saying it is up to the CWU members to end the dispute.
Mr Lynam said: "Those machines are available to be used and Sean is dragging An Post into this argument. An Post has no interest in shutting down the mail process, IO Systems has no interest.
"Those machines are available to be used today. If the CWU were to instruct their members to use those machines, they could be used immediately."