Ruairi Quinn was Irish finance minister from 1994-97 and was president of the Economic and Financial Affairs Council of the EU in 1996, working to introduce the single currency.
In a statement released by Better Together, the former Irish Labour Party leader said of a potential veto on Scottish admission to the EU from Spain and Belgium: "In my opinion, it is highly probable that at least two Member States, maybe more, will vote no."
He added: "If, however unlikely, Scotland was accepted as a full member it would have to agree that once its economy met the economic criteria of Economic and Monetary Union, as politically agreed in Dublin in December 1996, it would be legally obliged to join the Euro."
A spokesman for Yes Scotland said: "Mr Quinn will be aware that one of the entry criteria is membership of the exchange rate mechanism for two years - which is itself voluntary - so no country can be forced to join the euro against its will, as the example of Sweden makes abundantly clear."
He cited other experts who said that having had five million EU citizens for 40 years, Scotland would achieve a smooth transition to membership, adding: "That is sensible and in everybody's interest.
"The biggest threat to Scotland's continuing EU membership is David Cameron's in-out referendum which could see Scotland pulled out against our will."