The chief executive of aid agency Goal Barry Andrews has accused the European Union of "outsourcing its obligations" following its deal with Turkey.
Under the deal, all migrants and refugees, including Syrians, who cross to Greece illegally by sea from March 20th, will be sent back to Turkey once they are registered and their asylum claims have been processed.
The deal is expected to take effect from April 4th, by which time Greece must have in place a fast-track process for assessing asylum claims.
The EU has pledged to help Greece set up a task force of some 4,000 staff, including judges, interpreters, border guards and others to manage each case individually.
"The EU is outsourcing its obligations to a country outside the Geneva Convention," Mr Andrews told RTÉ's Morning Ireland.
He said he was at a loss to see how the plan is going to be implemented - even if it is legal.
“Over the weekend a thousand people came ashore on islands around Greece. On Saturday night alone there were 875. They have no idea of the new rules in force,” Mr Andrews said.
“These people aren’t aware of the changes and the EU staff on the ground don’t know how the rules are going to be implemented. It beggar’s belief that this is considered a solution.”
Mr Andrews went on to describe the proposal as “completely divorced from reality”.
“Only ten per cent of Syrian refugees in Turkey are in camps, the remaining 90 per cent are scattered throughout the community. Trying to re-settle 72,000 is a fantasy,” he said.