A Brexit would cause turmoil and radically alter relations between Britain and Ireland, a former president has warned.
Mary McAleese has also claimed Ireland's peace and prosperity would be in danger if Britain votes to leave the European Union next week.
She said: "Reassurances that nothing will change ... are wishful thinking at best and bluffing at worse."
Mrs McAleese, who was president from 1997 until 2011, will throw her weight firmly behind the Remain campaign as she launches a new report from the British Influence think tank at Westminster later.
In her speech she will urge British voters to avoid a choice for "drift" and loss of influence in Europe. She will also call for the 600,000 Irish citizens resident in the UK to vote stay.
She will also say that the benefits in Anglo-Irish relations, now taken for granted, could be put in peril while the future of 400,000 jobs and the open road border between Northern Ireland and the Republic would also be uncertain.
"The concerns of Ireland are legitimate and well-founded," she said.
"They involve the economy, trade, immigration controls, the hardening of the land border, security, the weakening over time of the excellent current relationship between Ireland and the United Kingdom, the impact on the peace process and the impact on European development of Britain's voice being absent from the European Union table."
Meanwhile, the British Influence paper, Brexit: The Irish Dimension, outlines the top seven problems that would upset Anglo-Irish relations in the event of Brexit:
They include the impact on Ireland's economy; disruption of the free movement Common Travel Area and the re-introduction of border controls as well as the ending of current extradition arrangements with the Republic of Ireland.
Other potential difficulties would be disruption to the peace process and the ending of EU-funded programmes while the all-Ireland electricity market and the energy relationship with the UK could also be affected, the report claims.
Peter Wilding, director of British Influence, said: "Vote Leave's 'It'll be all right on the night' attitude is playing with fire when it comes to the future of our relations with Ireland.
"Our report demonstrates - and Mary McAleese together with the entire Irish Government knows - that Brexit means trouble ahead for jobs, investment and the peaceful stability of the island of Ireland."
Meanwhile, Anthony Bailey, who sponsored the report, said he did not want to see the progress of the last two decades derailed.