It won't be the IRA that starts being violent, it will be loyalist paramilitaries. There are very well established links between NI loyalists and UK hard right groups.
In the event of a No deal brexit the level of anti-immigrant feeling is going to be very strong and you can expect groups like EDL to feel emboldened and create disturbances in places like Oldham, Bradford, London etc.
In Northern Ireland in the event of a no deal brexit the border between the republic and NI will become a massive issue. At the height of the troubles, with full army and special forces involvement the UK government could not control the border. It will be a massive free-for all of Human trafficking, drugs, high value produce and the criminal gangs that hide behind political ideology on both sides will be heavily involved. Inevitably a turf war will start and could escalate horribly quickly. The DUP will massively over-react to this and call for either more money, or direct policing support which would be disastrous. Even using UK border force would be controversial in the area, god forbid they ask the army!
Remember that events in NI can quickly spiral out of control. The initial British Army involvement in the 70's was to protect Irish catholic families from being fired out of their homes by loyalist mobs, but the presence of British forces on the streets created more problems.
I think this from Stephen Pound is quite telling