Why don't Sinn Féin take their seats in the UK parliament?
It seems that whatever happens (or happened) in a UK general election, the Irish nationalist party, Sinn Féin, will presumably always be in opposition.
This means that the British government - whoever they may be at the time - will always be happy with the fact that Sinn Féin don't take their seats - this time around, that's 7 opposing votes they don't have to worry about in parliament. So Sinn Féin is in effect half-supporting whoever is in power.
Surely they could do more damage to the government and further their own policies more by going to Westminster, even if it is quite a small minority.
What is the goal of this policy? What are they hoping the result will be?
If there are a total of, let's say 99 seats, and 50 are needed to pass a measure, not seating 7 still won't help the majority to reach that 50 vote threshold. Though, if it were some measure generally opposed by the ruling majority and there were enough defections, then the missing 7 votes would have more of an impact on something not getting passed. Or do measures pass by a majority of members who are seated in any session?
@PoloHoleSet: The latter. Measures pass if more votes were for it than against it. With 650 constituencies, an outright majority would require 326 MPs—but many measures are passed with fewer than that voting. That Sinn Féin do not take up their 7 seats means that measures can be certain of passing with only 322 MPs (but then, equally, they can be certain of being defeated with that many too—so it's a benefit to neither side).
The New Statesman sums it up fairly succinctly
Sinn Féin is an Irish republican political party active in both the Republic of Ireland and Northern Ireland. Its central aim is for a united Ireland. It opposes Westminster’s jurisdiction in Northern Ireland, and its oath to the Queen, so its MPs abstain from sitting in parliament.
In order to sit and vote in parliament, MPs must take the Oath of Allegiance
I, (Insert full name), do swear that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth II, her heirs and successors, according to law. So help me God.
While there is provision for atheist and agnostic MPs to affirm rather than invoking a deity they don't believe in, there is no provision to swear allegiance to an entity other than the British monarch, which Sinn Fein MPs refuse to do. In this sense, abstaining is the ethical thing to do. Having said that, it's unclear if Sinn Fein MPs would choose to sit in the House if the obligation to take the oath were waived, since they also believe the true government of Northern Ireland to be in Ireland, with the British an occupying power.
"there is provision for atheist and agnostic MPs to affirm rather than invoking a deity they don't believe in" Theists can affirm too (and do). Indeed, there was a period of time 1832-1888 when Christians could affirm, *e.g.*Joseph Pease), but atheists,like Charles Bradlaugh, weren't allowed.
There is another important reason. Arthur Griffith, the founder of the party, saw rejecting seats as a clear and powerful symbol of rejecting English sovereignty in Ireland. As a result, Sinn Fein run on an explicit platform of abstentionism. Even if the oath were changed or waived, they would thus refuse to take their seats because that's what their supporters voted for. They confirmed this during the negotiations following the 2017 hung parliament. This is pretty significant - I would consider detailing it an answer if this wasn't already accepted.
@owjburnham But you still have to swear allegiance to the Queen and the empire that occupies and enslaves your people. The DUP doesn't even want to recognize their language. No wonder they are upset.
@dan-klasson I know. I was just offering a correction/clarification on origimbo's answer, rather than attempting to answer the question.
@dan-klasson while I don't like the DUP, both their supporters and those of Sinn Fein were born and bred in Northern Ireland for multiple generations. To frame it as you have is somewhat one-sided and misleading. Also I'm not sure how 'their people' could be considered to be 'enslaved' in any conventional understanding of the word.
@DavidRicherby "Belief" is "that for which there is no evidence" (Bertrand Russell's definition, not mine!) If you have been indoctrinated since birth that you were enslaved by the outcome of the Battle of the Boyne in 1690, you aren't likely to change your opinion by thinking things out rationally. There is no more "logic" involved in the Irish Question than in the widely held attitude of many US citizens to the literal interpretation of their Constitution.