The leaders of Sinn Féin, the SNP and Plaid Cymru met in Cardiff on Monday and signed a memorandum of understanding committing their parties to cooperate on achieving constitutional change in Northern Ireland, Scotland and Wales. The meeting brought together Sinn Féin president Mary Lou McDonald, vice-president Michelle O'Neill, SNP leader John Swinney and Plaid Cymru leader Rhun ap Iorwerth.

The memorandum states the parties recognise each nation has the right to determine its own future through democratic means. It calls on the British government to prepare for, plan and facilitate constitutional change in each jurisdiction. It also outlines areas for practical cooperation including economic policy, renewable energy, relations with the European Union and self-determination.

O'Neill attended the meeting in her capacity as Sinn Féin vice-president, not as Northern Ireland's First Minister. Deputy First Minister Emma Little-Pengelly said O'Neill had no authority to sign anything on behalf of Northern Ireland or its Executive. Little-Pengelly said O'Neill was acting as a member of Sinn Féin and not representing the government of Northern Ireland.

DUP leader Gavin Robinson said O'Neill could not pretend to represent the Northern Ireland Executive or the joint office she holds with Little-Pengelly. UUP leader Jon Burrows described the event as stunt politics and said O'Neill was attending as a private citizen to sign a pledge that counted for nothing. SDLP MLA Matthew O'Toole said there was no issue with O'Neill meeting other party leaders and accused the DUP of being rich in its criticism given its past opposition to the equalisation of titles.

The meeting came two days after US President Donald Trump said during a visit to Ireland that he would love to see a united Ireland and it may as well happen now. Sinn Féin MP Pat Cullen welcomed Trump's comments but said she disagreed with him on many other policies. She said the conversation around Irish unity was now firmly on the table and called on Taoiseach Micheál Martin to begin preparations for a potential referendum.

Martin said he would raise reform of the first strand of the Belfast Agreement with British Prime Minister Andy Burnham, including the removal of a veto that allows Northern Irish parties to block the Assembly or Executive from functioning. He said most political parties in the North now support such reform.

A UK government spokesperson said the prime minister is focused on easing pressures on household finances and delivering economic security rather than engaging in constitutional debates or further referendums. After the party leaders' meeting, Swinney and ap Iorwerth signed a separate agreement between the Scottish and Welsh governments on cooperation in areas such as child poverty and cost of living.