Here's a piece about the Curragh Incident. Basically, the officer class of the British Army in Ireland said they would resign rather than fire on Unionists if those Unionists took military action when Home Rule was brought in.

The 30,000 British troops included those rushed in during the Rising, rather than only those present in Ireland at its start. And yes, Teribus you're right about the artillery being needed in France (if, of course, you think that the killing in France was right); Asquith's Liberal government fell in 1915 in a scandal over the lack of artillery shells available in France (the 'Shell Crisis') and he was left heading a cobbled-together coalition dominated by Ulster Unionists. So there was anger in Britain as well as in Ireland over the artillery used to destroy Dublin, though for different reasons.