Northern Ireland will be the "litmus test" for Brexit and businesses here say they remain "extremely concerned" about what sort of agreement will be secured.
Almost half of members of the Institute of Directors (IoD) in Northern Ireland say leaving the European Union will have a negative impact on investment, with just 12% believing things will improve.
Trade between Northern Ireland and the Republic is around £2.2bn each year, according to a major IoD report on Brexit.
Members are worried about "potential increases in regulatory burdens as a result of the UK's decision to leave the EU".
And the report highlights opportunities for businesses in the Republic to poach jobs from Northern Ireland companies.
One major concern is what potential border delays could have on trade.
"The UK leaving the European Union implies significant changes for these businesses and both countries' economies," the report says.
"In that sense, Northern Ireland will be the litmus test for Brexit.
"If the UK and Ireland can manage to overcome the many intricate and complicated difficulties Northern Ireland presents for Brexit, then the UK Government will have gone a long way to ensuring the best possible outcome of Brexit for the whole UK."
It raises concerns over the free movement of people and the potential loss of EU subsidies.
Ian Sheppard, IoD NI chairman, said: "Northern Ireland is unique as the only region of the UK having a land border with the EU. IoD members in NI are extremely concerned on what the impact of Brexit will mean for the free movement of people and goods across this border.
"It is vitally important the UK Government takes on board our concerns during the negotiating process and seeks to ensure that free movement is retained."
The report says some sectors here are "very heavily interdependent on these two islands". That includes agri-food and hospitality, which are likely to be hit hardest by border restrictions.
And on whether jobs from Northern Ireland could be lost to the Republic, the report says: "There are considerable opportunities for the Republic to lure some of these relocating jobs from the UK and, because of proximity, particularly those jobs currently located in Northern Ireland."