Irish foreign minister Charlie Flanagan, left, told the EU he is 'on their side'
Dublin told EU officials they had its unwavering support even though the UK is its most important trading partner.
Ministers also told Brussels they wanted to ensure that the border with Northern Ireland remains free after Britain quits the bloc.
The withering snub will not help Theresa May’s cause as she seeks to thrash out a post-Brexit trade deal with Brussels.
Ireland was considered one of the UK’s key allies around the EU negotiating table, given the countries’ close diplomatic and commercial links.
Theresa May has been hoping Ireland will be a key ally in the Brexit talks
Britain is Ireland's second biggest market for exports
But during a visit to the Belgian capital yesterday Irish foreign affairs minister Charlie Flanagan told the EU’s Brexit negotiators his government would row in behind their interests.
He confirmed that Dublin was “firmly on the side” of the remaining 27 member states as they attempt to ensure that the UK leaves the bloc on worse trading terms than it currently enjoys.
Mr Flanagan also issued an open invitation to Northern Irish voters, who backed Remain, to apply for Irish citizenship as a way of retaining their EU rights.
People who live in Northern Ireland are automatically eligible for citizenship of the Republic of Ireland. Successful applicants would still get to retain their British passports as dual-citizens.
We are firmly on the side of the EU27
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Irish foreign affairs minister Charlie Flanagan
He said: “We are firmly on the side of the EU27. I don’t see any positives in the withdrawal from the EU of the UK, for the EU or for the UK.”
Mr Flanagan continued that Brexit posed “monumental problems for the whole of Ireland”, but then added: “We don’t subscribe to the view that punishment should be exacted.”
He also reiterated the position of both the Irish and UK Governments that no hard border should be put in place between Ireland and Northern Ireland after Brexit.
Up to a million people cross the border every day, many for work, and it is feared that imposing checks and restrictions would have a devastating effect on local economies.
He said: “This border between north and south is invisible. We all want to maintain the invisibility of the border.”
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If you are an EU national, you do not need to show your national ID card or passport when you are travelling from one border-free Schengen EU country to another. The Schengen Area is an area comprising 26 European states that have officially abolished passport and any other type of border control at their mutual borders.
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Mr Flanagan met with chief EU negotiators Michel Barnier and Guy Verhofstadt as well as several prominent MEPs during a whistlestop tour to Brussels yesterday.
They discussed Ireland’s role in the upcoming Brexit talks, as Britain’s closest neighbour, with ongoing trade likely to be the number one issue for Dublin.
According to Observatory of Economic Complexity (OEC) figures the UK is Ireland’s second largest export market behind only the US, with Irish businesses selling us £15.4 billion in goods and services every year.
The country is also heavily reliant on Britain for incoming goods importing a third of everything it needs from the UK every year, to the tune of £19.5 billion.