I guess Brexit has energised all the home nations to seek either independence or reunification. I can't see why it should cause a return to violence though. I think the reasons for it starting up a long time back have changed sufficiently to prevent it happening.

The Irish government wants to keep the status quo on the border between Northern Ireland and the Republic of Ireland (the south). 

The border stretches 500 kilometres with 400 road crossings; about 1.8 million cars cross every month.  The Irish presently have an invisible border – it helps build the peace process as well as a functioning economy. Neither the north nor the south wants to be compelled by Brexit to impose cameras or physical stops to police customs, trade and illegal immigration. Yet Ireland still wants to allow the free movement of goods and services and people. 

If England can’t agree to the deal set by the EU, the result of a forced Brexit with no agreements, England would have to impose a hard border between the Northern Ireland and the Republic, in order to prevent a back door for invasion of illicit trade, disadvantageous trade, and illegal immigrants.

It’s doubtful that such a border would reignite the separatist terrorism of former years, but it probably would cause a great deal of inconvenience and hence tensions between relatives, friends and businesses on either side of the border.

But the “no deal” option would be disastrous for both England and Ireland. They need agreements on citizens’ rights, existing financial commitments, Irish sovereignty, trade and the timing and processes for the transition.