The owner of Patrick Pearse's handwritten surrender letter has accused the Dublin government of apathy over the unique 1916 artefact after it was taken out of the country.
The note, penned by the rebel leader in his prison cell to mark the end of the Rising, had been on display in the GPO in Dublin for over a year after a minister refused to pay the €1 million valuation and then blocked its export.
The US-based owner, who wishes to remain anonymous, paid €800,000 for it at auction in 2006 but said his original motivation was to ensure it stayed in Ireland.
He also said he put it back up for sale only to recoup his investment.
In a statement to the Press Association through a representative, the owner said: "The reaction of visitors who stare with reverence and respect has been a welcome riposte by the citizenry to official apathy."
Despite repeated approaches to government, no deal on a sale could be reached with the Department of Culture, Heritage and the Gaeltacht, prompting the owner to remove it from the GPO in the days before Christmas.
It is understood it was being hand delivered to New York on Wednesday after the one-year export ban lapsed.
The owner said: "I never sought to profit from my custodianship, I merely wished to recoup the cost of purchase when the time came to pass on the baton I picked up in 2006.
"Unfortunately, as there was nobody willing to take my place I will continue to protect and preserve this important national treasure albeit outside of Ireland now."
The letter failed to sell at auction in December 2016.
At the time the government insisted the guide price of between €1 million and €1.5 million (£840,000-£1.25 million) was too dear.
The Department of Culture, Heritage and the Gaeltacht said it would not disclose if it had been in talks with the owner to buy the letter.
"It is consistent departmental policy not to comment in any way on official interests or discussions relating to the possible acquisition of heritage items for the national collections, either by way of sale or auction," a spokeswoman said.
Just over a year ago the note was signed it into the Register of Cultural Objects, a tome of records held in Killarney, by then minister Heather Humphreys.
It is the first privately held item to have been put on the protective list and barred from being exported.
The owner said it was a fitting recognition for the cultural value of Pearse's letter.
But he added: "It is perhaps less fitting that in the national bank of cultural capital there are insufficient funds to keep it in Ireland now that the one year export ban has elapsed."
The owner explained his thinking behind buying the letter over a decade ago.
"I am not of Ireland but in 2006 I was moved by a piece of Irish history," he said.
"The final order to surrender, written by Patrick Pearse from his cell in 1916 are not just words on a page.
"The order is history brought to life in our hands.
"Irish lives were lost for the want of this letter, and Irish lives saved, and a nation was beget.
"The first faltering steps of a nation can be traced in every stroke of a pen.
"Mere knowledge of the words was not enough in 1916, the Volunteers required sight of the order, and my first sight of it moved me to buy it and keep it in Ireland."
It had been suggested that the owner could avail of a tax break to the valuation if it was donated to the state.
Pearse wrote the letter in his prison cell on April 30 1916.
Auctioneers regard it as one of the most historically significant artefacts from the rebellion years to have been offered publicly.
One of seven typed copies of the letter, produced by the British Army in the days after the Rising ended, sold at auction in London this year for close to €300,000.