Whether Mr McGuinness is or is not a member of the IRA is almost by the by. The fact is that he is almost universally perceived as one of the pivotal figures in the republican movement, a usefully nebulous term often employed as a euphemism to encompass both the legal Sinn Fein and the illegal IRA.
As a young man in the early 1970s, he was regularly described as leader of the IRA in Londonderry, building up a reputation as one of its most prominent street fighters in battles with troops in the Bogside and Creggan. In those days he was perceived as a straight militarist, with no hint that he might be interested in other forms of activity.
In personal terms, Mr McGuinness is known in Londonderry as a good family man, with something of a puritanical streak in matters of sexual morality. He is religious, though that does not extend to any acknowledgement that the Catholic Church has any jurisdiction over his political beliefs or his republican activities.
In 1972, he was again described as an IRA leader when he and other republicans, including Gerry Adams, were flown to London for secret talks with government ministers. He remembers that experience vividly: 'Gerry Adams and I went to London - it's like a lifetime away, 17 years ago, when we were only children really - and we talked to William Whitelaw.' He appears convinced that he will one day meet British ministers again, this time in effect to accept Britain's surrender.
What was slightly surprising was that a man with his reputation for straight militarism should have developed into such an enthusiastic advocate of building up the political party, Sinn Fein, alongside the IRA.
It was in the late 1970s that he, Mr Adams and other younger northern elements wrested control of the republican movement away from the southern-based old guard. In the early 1980s they brought Sinn Fein into the political arena. Since then he has become a local councillor and unsuccessfully stood several times for Westminster. Many republicans have worried that 'electoralism' might eventually soften the leadership's line and nudge it in the direction of a ceasefire. Mr McGuinness, with his particularly hawkish image, has in effect acted as an important guarantor to any doubters that Sinn Fein will not betray the IRA.
He must have his differences with Mr Adams, but the two men are careful to keep any disagreement hidden from public view. Many newspapers have sought to suggest splits between them, caricaturing Mr Adams as the dove and Mr McGuinness as the hawk, but they have shrugged off such portraits as 'black propaganda'.
A republican source said: 'Gerry is articulate, that's his strength. Martin has the respect and confidence of the volunteers. He's seen as their man. They trust him.' This was particularly important in 1986, when the disgruntled older guard split away from Sinn Fein.
Mr McGuinness assured the party Ard Fheis that the IRA went along with the Sinn Fein strategy. His crucial intervention was described thus by a Dublin magazine: 'McGuinness's speech was only thinly disguised as the authentic statement of the IRA army council.'
The security forces regard him as 'a very key player', while loyalists have plotted to kill him on at least one occasion. Last year he summed up his career by saying he had been to jail a number of times, had been stalked by an assassin 'and fired at by the British army on countless occasions over the last 20 years'.
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