A UK withdrawal from the European Union would impact on cross-border travel in Ireland, a report has said.
The so-called Brexit "could result in issues around freedom of movement in border areas," according to members of the Irish parliament.
It has called on London and Dublin to negotiate "a special position" for Northern Ireland, protecting north-south relations.
The UK government is committed to an in-out referendum by the end of 2017.
A report for the Oireachtas joint committee on European Union affairs recommends that the two governments "replace any lost EU funding for cross-border community initiatives".
Previously, a report for the Northern Ireland Assembly said any exit could cost the local economy £1bn.
The issue has divided Northern Ireland parties.
The two main unionist parties back a referendum based on a renegotiated deal.
The other three executive parties do not want change.
The report said the Republic of Ireland and the UK trade over 1bn euros-worth of goods and services every week and 25% of Northern Ireland's trade is with Ireland.
"The prospect of the UK leaving the EU is a very real one and the report emphasises the importance of Ireland being engaged on this issue from the very beginning," said committee chair Dominic Hannigan.
"The report concludes that that an EU without the UK weakens Ireland and Europe."