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4/ #NorthernIrelandLabourParty (#NILP): Belfast West by-election, 9 February 1943
https://en.wikipedia.org/wiki/1943_Belfast_West_by-election

The first and only Westminster seat won by the Northern Ireland Labour Party (NILP). The winner Jack Beattie later sat in Westminster for the Irish Labour Party, making Beattie the only MP to have sat in the UK House of Commons for any party which has ever held seats in Dáil Éireann.

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