Carolyn Fairbairn spoke after meeting business representatives in Northern Ireland and the Republic of Ireland.
She said: "Firms are making decisions now, they need to know now what sort of border solution is going to be found."
The operation of the Irish border is one of the most sensitive Brexit issues.
Political leaders in London, Dublin, Brussels and Belfast have all said there should be "no hard border" and "no return to the borders of the past".
However, so far there have been no firm proposals on how to achieve that when Northern Ireland is outside the customs union and single market and the Republic of Ireland remains inside.
The UK's Brexit Secretary David Davis said more time was spent on Irish issues on the first day of the Brexit talks compared to any other subject.
The Republic of Ireland's Foreign Minister Simon Coveney has suggested that Northern Ireland will require a "unique status".
He told the Irish Independent: "If we're going to avoid a hard border between Northern Ireland and Ireland, there needs to be some relationship with the customs union and common market that allows Northern Ireland to be able to operate the way that it does today."
Meanwhile, officials from HM Customs and Revenue are in Belfast for a series of meetings with business groups.
It is understood they are beginning a fact-finding process about border trade issues.
The director general of the Confederation of British Industry (CBI) has said an urgent solution is needed to Irish border issues after Brexit.