Finance Minister Michael Noonan has said that calls for a two-way deal between Ireland and the UK, to be negotiated before Brexit, could be illegal.
The idea has been put forward by the EU Committee of the UK House of Lords, which says it would be a good way of securing the future of Northern Ireland.
Their proposal would be for Ireland and the UK to agree a joint deal first, and then try to have that deal incorporated into the final Brexit agreement.
Minister Noonan says the plan could be illegal, however.
"We'll be part of the negotiating team under Mr [EU's chief Brexit negotiator Michel] Barnier, we have representation on it already," he said.
"So that's the official legal position.
"It's not possible then to sort out other difficulties which may emerge on the basis of a bilateral."