When UK ministers involved in the tortuous Northern Ireland peace process began talking to Sinn Fein leaders, they wondered privately whether the men across the table were former IRA “hard men” who had ordered or carried out killings, or mere civil rights activists.

As a former IRA commander, ministers had little doubt to which category Martin McGuinness belonged. Jonathan Powell, Tony Blair’s chief of staff, noted his “clear, chilling eyes”. It was odd for Blair to shake hands with men once banned from appearing on UK television – or, in McGuinness’s case, even prohibited from entering Britain.

McGuinness denied being the IRA’s chief of staff, although he admitted he had been a member. When he attended his first talks with Blair at Downing Street in 1997, along with his close friend Gerry Adams, the Sinn Fein President, McGuinness looked out from the Cabinet room to the garden where an IRA mortar bomb landed during a meeting of John Major’s Gulf War Cabinet six years earlier. He remarked: “So this is where all the damage was done.”

But Blair and his aides had read McGuinness wrong; he wasn’t talking about the IRA attack, as they believed. “No, I meant this [room] was where Michael Collins signed the treaty in 1921,” he explained, referring to the treaty of Irish independence that led to the Irish civil war. It was a sign that McGuinness knew his history but had also moved on to become a man committed to a lasting peace.

It is widely believed that Adams, although often seen as the senior partner in their Republican leadership but who was not an IRA member, could not have secured the IRA’s continuing support for the path to peace without McGuinness. He was instrumental in consulting key IRA figures at all stages to prevent the efforts to end the armed struggle ending in bloody failure as in the past.

McGuinness was a man with whom British politicians could build good personal relationships. He developed one with Powell, a key go-between in the peace process, joking with him about Unionist MPs, and the late Mo Mowlam, the then Northern Ireland Secretary, who called him “babe” and discussed her disputes with Blair.

McGuinness led the peace negotiations after apparently sending a message to the UK Government in 1993 offering a “total end to violence”. He told Blair in 1997 that Northern Ireland was a political rather than a security problem, saying the dispute could only be resolved politically, whether now or in 25 years.

The Good Friday Agreement was struck four months later. McGuinness was Education Minister in the Northern Ireland executive from 1999-2002, controversially scrapping the 11-plus, an exam he had failed as a child.

In 2007, McGuinness became Deputy First Minister in the devolved administration under the Rev Ian Paisley Sr, the Democratic Unionist Party leader. Their unlikely double act became even more remarkable as they became friends who respected each other.

In almost 10 years as Deputy First Minister, McGuinness’s energy was devoted to keeping the show on the road and proving that Sinn Fein could make the new devolved institutions work. He learnt to bite his lip when Unionists angered him; he did not think they were sufficiently committed to reconciliation. A key reason for his self-restraint was to build support for Sinn Fein in the Irish Republic, where he stood unsuccessfully for the President’s post in 2011, coming last in a three-horse race.

His relationship with Peter Robinson, who succeed Paisley in 2008, was less cordial than with Paisley. On a personal level it was better with Arlene Foster, who became First Minister in 2016. But their relationship soured over her role in a green energy scheme that hugely overspent, dubbed the “cash for ash” scandal. In January, McGuinness urged Foster to stand aside while an inquiry took place. She refused, and he pulled the plug on 10 years of devolved government by resigning, sparking the elections earlier this month to the Northern Ireland Assembly which have failed to end the stalemate and may result in the return of direct rule from Westminster.

Although McGuinness insisted his resignation was on political rather than health grounds, it was clear that his health was failing. He was suffering from amyloidosis, which attacks vital organs.

He announced that he would not stand in the elections in order to avoid the pressures of the campaign and to fight his illness. “I hopefully will overcome this illness through time. I am very determined to be an ambassador for peace, unity and reconciliation,” he said. “Reconciliation, I have always believed, is the next vital stage of the peace process.”

With the process now under threat, however strongly Unionists feel about McGuinness’s paramilitary past, they may miss his departure and the stability he brought to the table through experience and good judgement.