There has been a spike in demand for Brexit planning help at InterTradeIreland, the cross-border body said.
Its business monitor for the second quarter said that the number of firms on the island of Ireland reporting growth was at its highest level for seven years.
But their findings suggested businesses felt there remained "significant" challenges ahead.
At the same time, as a growing number of firms reported growth, there were also increasing numbers of firms reporting a decline.
There were mixed signals across the board, with uncertainty over Brexit weighing heavily.
Energy costs and other overheads, including wages, were major headaches for companies.
The going was even tougher for smaller firms and those in retail and construction.
However, over a quarter of professional services businesses were preparing for growth.
But one in four SMEs said they were putting off investment because of Brexit, while just under a third said they were concerned about sales. One in five had developed plans for when the UK leaves the EU.
Aidan Gough, InterTradeIreland's director of strategy and policy, said: "The number of firms preparing for Brexit is increasing all the time.
"We see this in the demand for our supports and services. In particular, our Brexit Advisory Service which offers bespoke help and assistance for SMEs, including a £2,000 Brexit start to plan voucher, has seen a big spike in demand.
"However, businesses can't afford to be complacent, as 80% of firms still have no plans to deal with Brexit."