Former Taoiseach Bertie Ahern says a potential exit from the EU by the United Kingdom would be "regressive" and "negative in every way".
Mr Ahern has rowed into the debate on a potential Brexit as the official 10 week period of campaigning gets underway.
He says a leave vote would have a significant impact in our border controls, in particular considering Ireland would "be the only place that has a land border with Britain - of course others would exploit and expose it. They'd have to check people".
Speaking to Sky News, the former Taoiseach added that he is concerned about the issues any such move from the UK would have on a cross-border trade, highlighting the need for a customs border to be established, and dubbing the move "regressive and negative".
However, the Northern Ireland Sectary Theresa Villiers says she believes the region would flourish if the UK voted to leave the European Union.
"We'd get back the right to run our own trade policy, that means being able to make deals with crucially important economies like the U.S., like China, like India".
Mrs Villiers, a Brexit campaigner, told Sky News' Murnaghan Show that leaving the EU would not alter the free flow of "goods and people" over the only land border with the UK.
"I believe that the land border with Ireland can remain as free-flowing after a Brexit vote as it is today," she said.
"There is no reason why we have to change the border arrangements in the event of a Brexit because they have been broadly consistent in the 100 years since the creation of Ireland as a separate state.
On Saturday, David Cameron was warned by Kenneth Clarke that he "wouldn't last 30 seconds" as prime minister should the voters choose to leave the European Union.
London mayor Boris Johnson, who is also backing the campaign to leave, stated that the campaign to stay in the EU was "depressing, as there is not a shred of idealism [in it]. Not a single one of them will stand up and admit that it is political. No one will say: ‘You know what, I love the idea of a federal Europe’, because that is the true logic of their position."
