I've been doing a little research into the Northern Irish Labour Party recently, just for the Archive. And reading up on Jack Beattie who was a former leader of the party, I found this:

When the 1949 Northern Ireland general election took place, the Anti-Partition League of Ireland was at its height. The middle ground in Northern Irish politics was squeezed. Beattie, who had accepted money from a fund established by the Anti-Partition League, "wore a steel helmet while campaigning in east Belfast". When one of his election meetings was broken up by an angry mob, he sent a telegram of protest to Downing Street which read "Stoned by official Unionist mobs and denied the right of free speech in my election campaign tonight. Armed Stormont police took no action".[3]

Beattie lost his seat, disbanded the Federation and joined the Irish Labour Party.[3] Tim Pat Coogan remarks that with the defeat of Beattie and other opposition candidates, "for the first time the opposition at Stormont was entirely Catholic, a matter of much satisfaction to [Prime Minister] Brooke".[5]

Although he lost Belfast West in the 1950 UK general election, Beattie retook it in 1951, losing it once more at the 1955 election. His attempt to win Belfast Central in the Stormont Parliament in 1953 also failed.

Does that mean technically the Westminster seat was held by the Irish Labour Party? On Wiki on the General Elections page for the constituency of Belfast West he is marked in as ILP. A fascinating constituency.