The Democratic Unionist Party (DUP) is the largest political party in Northern Ireland. Unionists are largely Protestant and fervently believe the six counties must remain part of the United Kingdom.

The party was founded in 1971 by the Rev. Ian Paisley – a firebrand preacher from the Free Presbyterian Church of Ulster.

Despite opposing power sharing with Irish nationalists for over three decades, in 2007 Rev Paisley became First Minister and Martin McGuinness – a one-time IRA member – became Deputy First Minister of Northern Ireland under the terms of the Belfast Agreement.

As it now seems clear the DUP will prop up the reeling Conservative government in Westminster, that accord demonstrates that while the party is rigid in its convictions it can also embrace a flexible pragmatism.

The current leader of the DUP is Arlene Foster.

Why do they matter?

The UK general election on June 8 delivered a hung parliament. That result was a shock, but by midday on Friday Teresa May announced a deal had been done to secure the support of the 10 DUP MPs.

Why was a deal so easy?

Previous Conservative governments have relied on Ulster unionist votes, but this is the first time a formal deal has been done with the DUP. Since 2015, when the Conservatives attained a House of Commons majority,  the two parties have developed a working relationship. This pact builds on that cooperation.

The Brexit factor

The DUP and the Conservative Party both back Brexit, despite the UK’s exit from Europe being rejected by its constituents at last year’s referendum. The DUP is determined to avoid any “special status” for Northern Ireland within the EU.  For unionists the link with Britain is key. Anything which alters the constitutional status of Northern Ireland within the UK is to be resisted.

The Corbyn Factor

UK Labour leader Jeremy Corbyn has openly supported the cause of a united Ireland and has ties to Sinn Féin and Irish republicans. Blocking Mr Corbyn from becoming UK Prime Minister is a DUP priority.

What do they stand for?

The DUP is extremely socially conservative. The party opposes same-sex marriage. It once opposed the decriminalisation of homosexuality in Northern Ireland with the slogan: “Save Ulster From Sodomy”.

The party is not pro-choice. Abortion laws in Northern Ireland are far more restrictive than in the rest of the UK and the DUP opposes any and all reforms.

Many in the party are sceptical of man-made climate change, especially of subsidies to renewable energy sources.

If you're wondering who the DUP are. Their manifesto is basically The Old Testament Bible with fortnightly bin collections.#dupcoalition

— 𝐏𝐞𝐚𝐫𝐥𝐲 (@pearlylondon) June 9, 2017

What does this mean for Northern Ireland?

The DUP and Sinn Féin had been sharing power in the Northern Ireland Assembly. That Executive collapsed in January 2017 and, following elections in March, no new Executive could be formed.

The role of the DUP in propping up the UK government will undoubtedly complicate negotiations between the Protestant and Catholic parties.

What can we expect?

The DUP will not take any cabinet positions. The deal will likely be one of “confidence and supply”, meaning the DUP will not veto supply bills nor back no-confidence motions, rather than a formal coalition of the sort represented by the long-standing Liberal-National coalition in Australia.

The DUP will prove to be tough negotiators — they always have been in the past.

During the Northern Irish peace process, they stood their ground against British prime ministers, Irish taoisigh (prime ministers) and US presidents. They also know their way around a pork barrel and will be expecting pet projects to be generously supported.

Dr Fergal Davis, Reader in Law, King’s College London, is a constitutional law expert who has been observing and writing about Northern Ireland’s law and politics for 20 years.