Ireland doesn't want to leave the EU. It's entire economy is based on EU membership. And they fought a war of independence in the 1920s specifically to avoid being in a 'mini union' with the UK. This kind of 'it would be easier if everyone did our bidding' thinking is one of the reasons exit negotiations with the EU have gone so badly.
The 'war', if it happens, would be a restart of the intractable civil kind among the various NI groups, not EU vs us.
I think Republic of Ireland not wanting to leave is "stopping them". They're much more integrated into Europe than the UK too. Have the euro for example.
Things would have been much less messy if Brexit had been implemented as a split of the EU, with the UK and Republic of Ireland running their own little 2-member EU government. Scotland could then be allowed to do their split from the UK while remaining in the mini-EU.