In Britain, labor groups throughout the country (from coal miners to railroad workers to dock workers) went on strike, culminating in unrest throughout the country worse than the “Great Unrest” of the early 1910s. Andrew Bonar Law’s Conservative-Independent Liberal government became increasingly weak, to the point that elections were needed in early 1920. British troops in Ireland were also increasingly bloodied, as the Irish populace turned against the harsh anti-revolutionary measures undertaken by the British. This all culminated in a landslide Labour victory in February 1920, giving Ramsay MacDonald (an original opponent of the war) a narrow majority in the House of Commons. MacDonald immediately pursued peace in Ireland.
1. Unless the timeline has changed things a lot (not really been reading it much, so might have missed summet) a Labour majority in the Commons as early as 1920 would have been impossible under just about any circumstances possible; there was no national Labour organisation to speak of before 1918 and the party was only active in a minority of constituencies.