Unionists have demanded answers after an Irish Tricolour was run up the main flagpole of Parliament Buildings at Stormont.
It is understood those responsible accessed the pole on the roof of the building in east Belfast without official permission.
The Republic’s national flag flew above the home of the North's devolved Assembly for around 10 minutes before it was removed. Another unsanctioned flag was erected on an adjacent pole at the same time the Tricolour was raised.
The PSNI said it was aware of the incident and was investigating the circumstances.
Major renovation work has been taking place on the roof this year so more people have had access than in normal circumstances.
Democratic Unionist MLA Peter Weir condemned what he branded a “provocative” act and called for an Assembly investigation.
“Whatever the motivation behind this there must be a full explanation from the Assembly as to who had access to the flag poles and who was responsible for this action,” he said.
“I have been assured by the Assembly that these were rogue actions and are being fully investigated.”
Ulster Unionist MP and MLA Tom Elliott said: “It was no doubt done to attract attention and cause offence and annoyance. The Ulster Unionist Party will be seeking a full explanation of how this could happen.
“We are perfectly clear that the Union Flag is the only flag that should fly over Stormont in order to reflect and respect the sovereignty of the United Kingdom.”
The Progressive Unionist Party has said the matter should be probed by the police.
But Sinn Féin accused unionists of overreacting.
Sinn Féin's Dáithi McKay tweeted: “Reaction of unionist politicians to Irish flag over Stormont a bit of a storm in a teacup. Need a flag policy that is inclusive not exclusive.”
The flying of flags on official buildings in the North is a sensitive issue.
In December 2012, a decision by Belfast City Council to limit the flying of the Union Flag over City Hall triggered months of loyalist protests, some of which descended into violence.
Like the policy adopted at City Hall, the Union flag is only flown on Parliament Buildings on a number of designated days each year.
But politicians have to date failed to agree a comprehensive framework for managing the flying of flags on public buildings and on lamp posts in republican and loyalist communities across Northern Ireland.