Ms O’Neill has described the decision to “unilaterally” appoint a special envoy to the US as “arrogant and disrespectful”.
Mr Ringland, a solicitor and community activist will be the first special envoy to promote Northern Ireland across the US administration, Congress and within the Irish American community.
As special envoy, Mr Ringland will also be tasked to work with US firms and investors with an interest in Northern Ireland.
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Mr Ringland said he was honoured to take up the role.
“I very much look forward to supporting the Secretary of State for Northern Ireland in ensuring the UK Government’s policies on Northern Ireland are well represented to US audiences and playing my part in forging deeper political, social and economic ties between our two countries,” he said.
However Ms O’Neill said: “Brandon Lewis’s decision to unilaterally make such an appointment directly relating to the North’s interests and priorities without consulting the democratically mandated Executive smacks of Tory arrogance and disrespect,” she said.
“That is not an acceptable way of doing business.
“The Executive has a permanent diplomatic mission based in Washington DC which comprises civil servants under the auspices of the Executive Office.
“It has proven to be effective and we do not need interference from the Tories in how we engage with the US administration, Congress, or the Irish-American community.
“I consider this announcement to be yet another attempt by this Tory Government to undermine devolution, locally elected ministers and our institutions of government.
“This is in addition to their announcement by a Whitehall department to establish an office in Belfast, which is frankly needless.
“I intend raising these concerns with the British Secretary of State at the earliest opportunity. No power-grab or undermining of devolution will be entertained.”
Earlier Northern Ireland Secretary Brandon Lewis said: “I am delighted that Trevor has accepted my invitation to take up this position to represent the UK Government in Northern Ireland, working to support its best interests in the United States.
“This is an exciting new appointment and recognises Northern Ireland’s unique relationship with the United States and its people. I look forward to working with Trevor to harness all the opportunities that lie ahead.”
UUP leader Doug Beattie said the creation of the role is “great news and provides fantastic opportunities”.
“The Government’s appointment of Mr Ringland in that role is a very positive step forward and I hope everyone gives him their support as his work begins,” Mr Beattie said.
“Trevor has a unique understanding of relationships across communities and business in Northern Ireland.
“He is a strong advocate for the Union and a unionism that is all-encompassing and reaches out across the divide.
“Everybody should be able to get behind him and support him in his work as he promotes Northern Ireland and its people across the United States.
“Northern Ireland has so many links with the United States – socially, historically, culturally and economically – and I am delighted that we will now be making more of the opportunities that those links provide.
“We need more engagement, not less.”