STORMONT civil servants granted Sinn Féin access to the Maze prison site despite the party blocking its use in a spat with the DUP spanning six years.
Since 2013 the vast majority of requests to use the site have not been approved because of an ongoing row between the parties over abandoned plans to build a peace centre.
The DUP's Peter Robinson, who was first minister at the time, halted £300 million redevelopment plans for the site after unionist critics argued it would become a shrine to terrorism.
Sinn Féin's Martin McGuinness, then deputy first minister, responded by saying no further development would take place until the issue was resolved.
Bids from most groups including charities and film crews to use or visit the site near Lisburn have been blocked since then, as it required approval from both the first and deputy first ministers.
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At the time the DUP blamed Sinn Féin, while Sinn Féin blamed the DUP for "reneging on its commitment".
The Maze/Long Kesh (MLK) prison was the site of the IRA hunger strikes and held some of the north's most notorious paramilitaries before it closed in 2000.
Since the Stormont executive's collapse in 2017, civil servants have maintained a block on most requests to use the site.
Their only known exception was last year when the Ulster Aviation Society (UAS) was allowed to hold an open day in September after years of failed attempts to gain approval.
But it has now emerged that last July, Sinn Féin was also granted permission to visit the site.
The party's access to the former prison site was revealed in a Freedom of Information response to The Irish News.
Fifteen other requests to access the site during 2017 and 2018 were not approved.
Ulster Unionist MLA Mike Nesbitt said it was "quite extraordinary" that Sinn Féin were "among the chosen few who have been given access".
The former UUP leader said Sinn Féin blocked UAS from using the site "in a huff" over the peace centre plans.
"I see no reason why that bad behaviour should be rewarded with one of the very few entrance tickets handed out in recent years," he said.
Mr Nesbitt added: "It is my understanding that civil servants continue to operate policies that were in place with ministerial approval before devolution collapsed. In this case, that means they should have asked the DUP if they were content for the Sinn Féin delegation to visit," he said.
"Either they did not ask the DUP – which would be to overturn the previous policy – or they did ask and the DUP said yes. Either would be truly remarkable."
SDLP Lagan Valley MLA Pat Catney said: "The SDLP has always supported the development of the Maze/Long Kesh site in a way that sensitively and objectively conveys its history and generates economic investment in the local community.
"It is incredible, however, that Sinn Féin applied and was granted access to the site given that they have spent years vetoing its use by any other charity, university or media outlet.
"They have some brass neck."
A Sinn Féin spokeswoman said: "Sinn Féin visited the site along with an architect to ensure the site and its listed buildings were being maintained."
The DUP did not respond to requests for a comment.
The Executive Office was asked several questions, including why officials granted Sinn Féin access to the Maze site while not approving others, and whether this meant other groups would now be granted use of the site.
In a single-line statement, a spokesman said: "There has been no change to the Maze/Long Kesh Site event approval process."
