Dublin city councillors have voted against a land swap deal at Dublin's historic Moore Street.
The move would have seen a restoration of the 1916 Easter Rising site in time for centenary celebrations.
The buildings were used by 1916 rebels - Padraig Pearse, James Connolly, Joseph Mary Plunkett, Sean Mac Dermott and others - when they supposedly retreated to the building after they abandoned the GPO.
Officials had earlier proposed accepting an offer from developer Chartered Land to restore the national monument at 14-17 Moore Street.
This would ultimately have meant that the city council would receive ownership of the site together with a fully completed heritage centre by Easter 2016.
The developers had adjusted their offer to include No. 10 in the deal.
However, councillors voted not to accept the land swap proposal at last night's council meeting.
The decision by city councillors to reject the plan was described as an "insult" to the memory of all involved in 1916 by independent councillor Nial Ring.
Cllr Ring, who also chaired the Moore Street Advisory Committee, said that by voting against the deal, councillors had lost a unique opportunity for the city.
Cllr Ring's grandfather and four granduncles were part of the GPO garrison that evacuated from the GPO into Moore Street.
"The current situation is that the National Monument site will be left as it is because the owner can separately purchase the site needed at 24/25 Moore Street, has a valid planning permission to build a shopping centre and has absolutely no obligation, legal, moral or otherwise, to build a heritage centre," explained Cllr Ring.
"By DCC owning and operating the heritage centre, guaranteeing to be completed by 2016 with DCC influence on design, layout, contents etc, any future planning would have been influenced by the existence of a fully restored and operational heritage centre - but that opportunity has now been lost," he continued.
Cllr Ring said that the decision has thrown the memorial plans for the 1916 centenary into disarray.
"Anyone who believes the centre will go ahead after 2016 is out of touch with reality, a reality which will see the GPO 1916 centre, Richmond Barracks restored and Kilmainham Jail extended in time for 2016.
"For whatever political reasons some councillors have turned their backs on our recommendations and have turned their backs on 1916," Cllr Ring concluded.