Theresa May is lurching from one crisis to the next during her shambolic leadership. The summer recess may provide some respite for the British prime minister, but when she returns to the Commons in the autumn, she will face more important votes on Brexit.

There are growing numbers of rebels from within her own party, both from hard Brexiteers and Remainers. Last week, Ms May avoided defeat in the Commons in a crucial vote on leaving the customs union by a margin of six.

Sinn Féin has seven MPs who currently do not take their seats at Westminster. If this anachronistic policy was abandoned, Sinn Féin could fight for Irish interests where the biggest decisions are being thrashed out regarding Brexit — the House of Commons.

Abstentionism — when elected representatives refuse to take their seats in parliament — is nearly as old as Sinn Féin, which was founded in 1904 by Arthur Griffith.

When the fledgling party pledged to voters that their representatives wouldn’t take their seats at Westminster, it didn’t really matter, as the party only produced a tiny number of MPs. The Irish Parliamentary Party (IPP) played the game of parliamentary arithmetic in its failed attempt to win Home Rule for Ireland.

However, the policy became exceedingly useful in 1918, when the tide of public opinion in Ireland turned from support for the IPP to Sinn Féin. Refusing to go to Westminster en masse, Sinn Féins’ 73 MPs stayed at home to form their own assembly.

The first Dáil met in January 1919, the same day as the opening shots of the War of Independence were fired. By fighting the British army and establishing a counter state through political organisations such as the Dáil Courts, Irish independence was achieved in 1922.

Those anti-treaty Sinn Féin TDs who refused to recognise the legitimacy of the new state also abstained from the new parliament, but their numbers dwindled and the lost ground to the new state builders in Cumann na nGaedheal.

De Valera was pragmatic enough to recognise that the policy was leading them nowhere; he established Fianna Fáil in 1926 and entered parliament the next year.

There were purists who continued to abstain, like the rebel priest Fr Michael O’ Flanagan, who became president of Sinn Féin in 1933. Sinn Féin languished in the middle decades of the twentieth century as Fianna Fáil dominated Irish political life.

The party was rejuvenated by the onset of the troubles, reforming as Provisional Sinn Féin in 1970, though it remained very much the poorer and smaller cousin of the Provisional IRA. Abstentionism continued as a core policy of the party, but the Republican movement’s main efforts were focused on the war.

However, with the election of Bobby Sands to the House of Commons in 1981, Sinn Féin began to recognise that it may become expedient for them to drop abstentionism. Sands had sidestepped the abstentionism issue by running as an ‘Anti H-Block/Armagh Political Prisoner’ candidate.

Skilful manoeuvring by Gerry Adams and his allies in the Republican movement led to abstentionism being abandoned in the south in 1986. Sinn Féin performed poorly in elections, with IRA’s war producing too much bad press for its political wing, and so Adams wound down the war to focus on electoral politics.

Thirty-two years after dropping abstentionism in the south, Sinn Féin is a rising force. It provides effective opposition and may make up part of the next government. This obviously would have been impossible without dropping abstentionism.

This brings us to its usefulness in the North.

Whether one loves or loathes the EU, Brexit presents the biggest threat to Irish stability since The Troubles. Key votes have and will be happening in the Commons over the coming months. Ms May is stumbling on with her Brexit plans in tatters.

The DUP cannot be trusted to fight for issues which will affect the whole island. Ms May only passed Brexit legislation with the narrowest of margins. If Sinn Féin MPs were at Westminster, they could influence the shape of Brexit and the course of Irish history.

When questioned, Sinn Féin MPs say that they were elected on an abstentionist mandate.

However, they have dropped it in the past when it became politically useful. Now, surely, is one of those times. One of the reasons for Sinn Féin’s continued success is its ability to adapt and remain relevant in a changing Ireland.

It is time for Sinn Féin to drop this symbolic protest, enter the House of Commons, and fight for Ireland’s interest.

Fergus O’Farrell is a writer and historian based in London. His work has appeared in History Ireland, the Irish Times, and the Irish Independent. His biography of Cathal Brugha will be published this September by UCD Press.