But if the army rebelled en masse, how would it be a civil war? Wouldn't it just be a coup?
As PCH said, that assumes that the entire army would rebel. It wouldn't. Some regiments would revolt against the government, others would fight for them against the government, and others would back out completely. Some would fight for Home Rule, some against Home Rule, some against the Irish, some for the Irish, and some would follow the government no matter who ruled, while others would switch sides when the government changed (as it would have).
Also, PCH mentioned that the British Army wasn't very large at this point, which is correct. It was only during WWI that the British Army ballooned, and even then most of the troops were colonials. And the British couldn't rely on those colonials to back them over the opposition, meaning that they'd probably avoid using them until they didn't have a choice.
I don't think anyone's figured out exactly how a civil war in Britain would have effected the colonies, but you'd likely see confusion and rioting in Australia and New Zealand, and quite probably open rebellion in South Africa by the Boers. India was more complicated, but it was also what the British were most afraid of losing, so they'd avoid using Indian troops even more than troops from elsewhere in the Empire. Canada and Africa were the best places to get troops from for pacifying Britain itself, and the troop numbers there weren't exactly huge.Not to mention the backlash against the government if black Africans were used to fight their own people.
Have to say, I had no idea whatsoever that it was such a major threat to the entire British state, I really thought it was a fairly localised thing. Then again, I'm singularly ignorant on the parts of Irish history my family didn't take part in.
The British have never admitted exactly how desperate the situation was in 1914. That would mean admitting that the official reasoning behind WWI was bullcrap, which would mean admitting that the war wasn't Germany's fault, which would mean admitting many other embarrassing things. Not the least of which would be that Winston Churchill was openly preparing to turn government soldiers on
British citizens - no-one cares about using them on the Irish - in the event of civil war. That would be extremely embarrassing for the Tories, especially since Winston was Prime Minister on more than one occasion.