Can’t have been easy for Martin to that Royal Banquet. He’s never struck me as the Black Tie type. But clearly there was method in his constituional madness. One, not going would have been a step back from the last time. Doing it, as Tom Kelly points out allows them to project a statesman like image:
The debacle over the flying of the Union flag at Belfast City Hall hurt many rank and file unionists and whilst Alliance got most of the blame for letting it happen, Sinn Fein who like to engage in gesture politics has been looking for opportunities ever since to assuage ordinary unionists that they recognise their Britishness. So the first ever-official state visit by an Irish President to Britain is just the type of opportunity they have been looking for. The current tetchiness of the strained relationship between Peter Robinson and Martin McGuinness also gave Sinn Fein an opportunity to present their leadership as statesman like.
Sinn Fein is also likely to have electorally calculated that whilst they will remain the largest nationalist player in the North- that their support levels may have actually plateaued. So the real game changer for Sinn Fein is in the Republic of Ireland where the party is eradicating the working class base of the Irish Labour Party. Current polls suggest that the next Irish election may see a proper left-right re-alignment and the goal of Sinn Fein is to at least make them seem like a suitable partner in a coalition government.