I posted earlier in the thread about this, the general consensus here is a preference for the UK to remain in. The potential for disruption the current level of trade between the two States is too great with an Exit.
Another unknown is the attitude of an future UK government with regard to its only land border with the EU. If the UK is happy to retain the common travel area, then I can't see any issues. However if immigration becomes a hot issue, I can see the border hardening. This will have political (or worse) consequences in the North.
Another unknown is the status of Irish nationals living in the UK and Vice versa in the event of an Exit.
Nate