By Mark Devenport
Northern Ireland political editor
MPs from Ireland and Northern Ireland played an important role during the finely balanced parliaments of the 20th century.
If the 2010 general election produces the first hung parliament of the 21st century, they could once again find themselves in an influential position.
During the minority government of 1910-1915, Irish nationalist MPs kept the Liberals in government in return for progress on Irish Home Rule.
In 1974, the Ulster Unionists rejected an offer of entering a coalition with Edward Heath's Conservatives.
In 1977, the minority Labour Prime Minister Jim Callaghan had discussions with the Ulster Unionist leader Jim Molyneaux which played a part in Northern Ireland's representation in the Commons eventually rising from 12 to 17.
Peter Robinson's DUP are the biggest NI party
Then in 1979, the independent republican MP Frank Maguire played a key role in the fall of Jim Callaghan when he famously attended a Commons no confidence debate in order to "abstain in person".
Going into this year's general election, Peter Robinson's Democratic Unionists are the biggest Northern Ireland party.
After the revelation of secret talks between the DUP, the Ulster Unionists and the Conservatives at Hatfield House in England in January, DUP sources pointed out that their party's voting record was heavily in favour of the Tories.
However in June 2008, the DUP saved Gordon Brown by backing the Labour leadership over its plans for the 42 day detention of terror suspects.
The former DUP MP Iris Robinson memorably held nine fingers up to the Conservatives (one for each DUP MP) to signify their influence in a tight vote.
In a post election scenario the DUP might play hardball with the Conservatives, not least because David Cameron has formed an electoral pact with their principal rivals the Ulster Unionists.
However, their participation in the Hatfield House talks shows they are willing to be flexible if their concerns are met.
Lady Sylvia Hermon disagreed with her party's alliance with David Cameron
The Conservatives hope their "new force" with the Ulster Unionists will deliver them two or three valuable votes in a hung parliament.
Conservative sources claim they are neck and neck with the DUP in target seats like South Antrim and Strangford.
If any Conservative and Unionist candidate succeeds they will take the Tory whip, so their votes can be considered "in the bag".
However, the DUP points to recent government opinion polls which suggest they remain well in the lead.
In March, the sole MP representing the Ulster Unionist Party, Lady Sylvia Hermon, resigned from the party and will contest the election as an independent.
She said she was leaving the party, having refused to support its pact with the Conservatives.
Lady Hermon has always maintained good relations with Labour and if she did get re-elected it is likely she would back a Gordon Brown led government.
The other unionist party in the fray, the Traditional Unionists, do not have a sitting MP.
Gerry Adams and his colleagues refuse to take their seats at Westminster
However their leader, former DUP MEP Jim Allister, generally adopts a right of centre approach to issues like European integration or taxation.
On the nationalist side, the three outgoing Social Democratic and Labour MPs generally follow the Labour whip, although there have been occasions (such as the 42 day detention vote) on which they have joined Labour rebels in siding against Gordon Brown.
But when it comes to the formation of a new government, any SDLP MPs elected in 2010 can be relied upon to back their sister Labour party.
That leaves Sinn Fein, who have five outgoing MPs.
True to their abstentionist Irish republican creed, Gerry Adams and his colleagues refuse to take their seats at Westminster, although they do receive financial allowances to cover them for their constituency work.
Sinn Fein insists their stance will not change no matter how tight the result of the 2010 election might be.
But even if they do not, like Frank Maguire in the 1970s "abstain in person", their absence will still influence the parliamentary equation as it will lower the mathematical margin which any would-be prime minister needs to reach in order to form a new government.