Why has Northern Ireland had more of a history of violent conflict in the last few decades than Scotland, Wales, or Quebec, even though those last three have all had sometimes-acrimonious nationalist disputes?
While I'm far from an expert on the Quebec situation, I could give a few reasons for Northern Ireland, Scotland and Wales. Firstly because Irish rebels had actually gained success by establishing the Irish free state in 1922 and the Irish republic in 1937, they have more motivation than the Welsh and Scottish who have rarely made any progress at all apart from minor political rights. (Although this could change later this year with the Scottish independence referendum. http://www.scotreferendum.com/ Partly due to the Independence those rebels have been able to survive and grow their operations this time with a base in a country in support of their activities. ( This is evident in the trials of Charles Haughey and Neil Blaney, two politicians found innocent of arms traffficking in a southern court even against overwhelming evidence and international pressure). Those rebels in the past and in the last few decades have preferred guerrilla style warfare to the open opposition characterized by Scots such as Robert the Bruce but are larger scale than the Welsh nationalists.