The Grand Orange Lodge of Ireland has withdrawn from the Shared Island Unit, set up by the Republic's coalition government to foster good relations, as anger continues to grow among unionists at the Northern Ireland Protocol.
In a letter sent to Taoiseach Micheál Martin on Wednesday, the loyalist fraternity accused the Irish government of showing "scant regard for the views of the unionist community".
"The Orange family will not play any part in a process that masquerades as two neighbouring jurisdictions improving relationships for the benefit of both countries when in reality they pursue an agenda that enforces a Protocol that normalises and 'talks up' the inevitability of a United Ireland," the letter states.
The Irish Government's Shared Island initiative, with a budget of €500m, was set up to work with the Northern Ireland Executive and the British Government to address strategic challenges faced on the island.
It involves unionist outreach and fostering "constructive and inclusive dialogue and a comprehensive programme of research to support the building of consensus around a shared future on the island".
However, withdrawal of support from the largest unionist group involved in the Shared Island project throws the future of the initiative into doubt.
Rev Mervyn Gibson, grand secretary of the Orange Order of Ireland said the letter was in response to the mood among members "getting worse by the day as people realise the reality of what the Protocol means".
"We have withdrawn any engagement with the Shared Island Unit, because it is clear that the government in the Republic is trying to ride two horses, and therefore it has no longer had any genuine outcome.
"People are beginning to see the full implications of what the Protocol means, and I pray and I trust people will not turn to violence, but if we'd no restrictions I think there would be protests on the streets, people are being pushed into a corner."
The move by the Orange Order, which has over 1,000 lodges across the island, will further intensify pressure on both the British and Irish governments to find compromises on Brexit trade arrangements.
Earlier this month the Loyalist Communities Council (LCC), an organisation that represents the views of the UDA, UVF and Red Hand Commando, said in a letter to Boris Johnson that the paramilitary groups would be withdrawing support for the Good Friday Agreement in protest at the protocol. The Orange Order's withdrawal from the Taoiseach's Shared Island unit is designed to send a message to the heart of the Irish government that unionist's anger at the Irish Sea border is growing.
"The current situation regarding the Protocol has, in our opinion, fundamentally damaged relations between Northern Ireland and the Republic of Ireland", the letter states.
"The attitude, actions, and words of members of the Irish Government have shown scant regard for the views of the unionist community in Northern Ireland and an extremely selective regard for the workings of the Belfast Agreement.
"The assurances by Micheál Martin that he sought to work with all to build consensus around a shared future underpinned by the Good Friday Agreement, increasingly rings hollow with our members.
"When the threat of a resurgence of border violence was introduced into the Brexit debate in Europe and talked up by Leo Varadkar, (touting a photo of a Customs Post destroyed by 'Republican' terrorists) and when the ongoing unhelpful comments of Simon Coveney over recent times are placed alongside the strong-arm tactics of your allies in the EU and US it leaves little room for hope that there is any meaningful interest in understanding, let alone addressing, the concerns of our community.
"As an organisation affected by the violence of the past, but which nonetheless had embarked on a journey to build good relations with our neighbours, we feel deeply disappointed by the Irish government's actions in recent weeks and months. We feel that the building blocks of mutual respect and co-operation have been replaced by an attitude of arrogance and indifference."
A spokesperson for the Irish Government said: "The correspondence referred to has not been received as yet.
"The Government has engagement with the Orange Order in a number of areas, and would wish to maintain and further develop that in the period ahead".