ISME fears black economy could be worth up to €461m a week to rogue traders.
The Small and Medium Enterprises association believes there's been a surge in illicit activity and "nixers".
ISME blames the high costs of administration and regulation in Ireland and claims if it could be reversed the exchequer could pull in up to €4.8bn.
Roubini says big banks need to be broken up.
The economist once dubbed "Dr Doom" by the New York Times for his pessimistic, but ultimately accurate predictions about the global economy at an IMF meeting in 2006, was speaking in Dublin yesterday.
He believes Ireland is storing up problems for the future by enabling consolidation within the banking sector.
"If a bank is too big to fail. It's too big," he said.