THERE IS nothing new about politicians from Ireland playing a pivotal role in the affairs of Great Britain. It is a pattern which has recurred in various forms for two centuries. Whether they were Irish nationalists or advocates of union with Britain, elected representatives from the smaller island have often tilted the balance of power in Westminster. The current situation, with the Democratic Unionist Party (DUP) preparing to prop up a minority Conservative government, is only the latest variation on that theme.

Ireland began sending members to the Parliament in London in 1801 after its own unrepresentative legislature was closed under the Act of Union between the two islands. At first the Irish members were an occasional wild card in British House of Commons. In 1873 Irish parliamentarians helped bring about the eventual defeat of a Liberal government by killing a bill on a secular university system for their home island. But after 1885, when Liberal leaders backed Home Rule for Ireland, the Irish question (ie, whether or not Ireland should govern itself) became the defining factor in Westminister’s contests, and Irish influence rose correspondingly. For 30 years, British politics turned into a battle between advocates of Irish-British union (Conservatives and some former Liberals), and pro-Home Rule Liberals who were backed, often decisively, by a caucus of around 80 nationalists from Ireland.

But the influence of Irish nationalists in the House of Commons was especially strong after elections in 1892 and 1910, when they held the balance of power. They used their political heft to press their Liberal allies to advance Home Rule and overcome resistance from the House of Lords. Meanwhile Unionists in Britain and Ireland galvanised each other. Sometimes it was London’s politicians who stirred up pro-British sentiment among the Protestants of Ireland; in 1886 Lord Randolph Churchill made a famous speech predicting that to avoid all-Ireland Home Rule, “Ulster will fight and Ulster will be right.“ At other times, it was the Irish Unionists who pressed their Conservative friends in London to stand firm. In 1903, when a Conservative government made concessions to Irish nationalism, the Ulster Unionists objected.

From 1922 onwards, most of Ireland became independent, and only the six counties of the north-east, which remained in the United Kingdom, sent members the House of Commons. There were a dozen of them and the great majority were Unionists who sat and voted with the Tories, whose full name had become the Conservative and Unionist Party. In the 1960s a Labour government voiced frustration over this, fearing that Unionist votes would block nationalisation policies. But after 1972, when a Conservative government alienated its Unionist friends by closing the Northern Irish Parliament, Unionists sitting in Westminister were estranged from both of the main British parties yet open to deals with either. In 1977 they propped up a Labour government in return for an increase in the number of Northern Irish seats in Westminster. (There are now 18.) In 1993 a Tory government led by John Major parleyed with the Unionists to gain crucial support for a bill on a European treaty. A more recent Tory prime minister, David Cameron, showed interest in reviving his party’s ties with the Unionists. His government discreetly mooted the idea of an electoral pact between the Tories, the moderate Ulster Unionists and the harder-line DUP. Still, for a British government to be openly dependent on a deal with the DUP is something new. Another new development is the complete absence of moderate Irish-nationalist voices from the House of Commons, with the electoral defeat of all three members of the Social Democratic and Labour Party. Sinn Fein, representing militant Irish nationalism, will refuse to take up the seven Westminster seats it has won, reflecting a long-standing policy of keeping Irish and British politics completely separate.