A British withdrawal from the European Union would cost Ireland billions of euro every year in lost trade and hiked energy costs, a new study has found.
The analysis by the Economic and Social Research Institute also suggests business in Northern Ireland and along the border would be worst hit by a so-called Brexit.
The ESRI looked at the potential economic consequences for trade, foreign direct investment, energy and migration.
"The bottom line is the Irish interest is not served by the UK leaving the EU," said Dr Edgar Morgenroth, associate research professor at the ESRI.
The report author said a post-Brexit drop in Irish exports to the UK could cost as much as €3bn every year to the economy.
Home-grown firms would take the brunt as they rely more heavily on trading over the Irish Sea, compared to the multinationals.
Firms along the border would also be badly affected as they rely on sales to Northern Ireland.
But Dr Morgenroth also pointed out that businesses in Northern Ireland would be harder hit by a demise in North/South trade, as they gain more from the relationship overall.
"Given that there seems to be a bigger reliance on the Irish market for Northern Ireland businesses, any trade impediment would obviously hurt Northern Ireland more," he said.
It is estimated both the Republic and Northern Ireland would suffer a 20% drop in trade.
The study also looked at the impact of a Brexit on the energy market, the cost of which could be upwards of €10bn.
It found that the Republic could be forced to look at building a new electricity interconnector with mainland Europe, most likely France, if current access through the UK was blocked.
Commenting further about potential energy issues on RTÉ's Morning Ireland, Dr Morgenroth said: "We have an all Ireland electricity market. Having an external border with a non-EU member state might make that much more tricky.
"We're also connected to the UK on the energy side in terms of gas, we get 100% of gas from the UK. And we have only a connection through the UK to the wider EU electricity markets."
The report also scotched suggestions Ireland could significantly benefit from more foreign investment by global firms by-passing the UK in favour of Ireland, as it remains in the EU.
"Our analysis suggests this would not be significant," said Dr Morgenroth.
"Bigger countries like France and Germany would probably benefit more from a redirection of foreign direct investment away from the UK."
