Strangely, the immigration aspect between Ireland and UK is of less relevance, the points of entry to UK mainland are actually not that many and have been monitored for a good few years, so get into NI is the easy part.
The leave people point out that free movement has existed for almost 100 years, which is true.
Free movement of goods is another thing, a colleague yesterday reminisced about his holiday, as a UK citizen no worry about passport, ID, just hitch up the caravan and set off for the lush greens of the Republic of Ireland.
Got stopped at customs, needed a 'carnet de passage' for the caravan, took about 2 hours, rules said he had to check back in on the way home - i.e. he didn't leave the caravan behind him.
In the absence of a trade agreement such rules may apply, ok, so he didn't bother to check in on the way back, but business has to apply the rules.
My own feeling is that it boils down to which is the more important, movement of people or movement of money - also think that both go together.
Btw, one finance global that I have a little contact with have placed their UK expansion on hold, they figure there is no need for FSA regulation if UK is outside, already now in Dublin as the only other major English speaking - the Dubs are working hard on the fear factor, je crois.
Another btw, 'Eire' is usually consigned to history, seldom in the modern day Republic of Ireland, member of the EU