The impounding of two Northern Ireland fishing boats by the Irish Navy has been condemned as “quite outrageous” by the DUP deputy leader Nigel Dodds.

The boats, the Amity and the Boy Joseph, were detained in Dundalk Bay on Wednesday for allegedly breaching fishing regulations. They were fishing for crabs, lobsters and whelks when they were arrested.

In a statement, the Department of Defence said the ships were detained for illegal fishing within the exclusive fishery limits of the State and for illegal entry into the limits. The two boats were escorted to Clogherhead in Co Louth by a fisheries protection vessel and handed over to gardaí.

Northern Irish boats are currently prohibited from fishing inside the Republic of Ireland’s territorial limit while vessels from the Republic can fish in Northern Ireland waters. Up until 2016, under the “Voisinage agreement” dating back to the 1960s, there was a “gentleman’s agreement” where fishing vessels from both sides of the Border enjoyed reciprocal rights in Irish territorial waters.

However, the deal broke down following a successful legal challenge to the agreement in the Supreme Court in Dublin in 2016. The court ruled Voisinage was an informal agreement of insufficient legal standing to formally grant access to foreign-registered boats.

That decision effectively banned Northern Ireland boats from fishing in Irish inshore waters . The UK has continued to recognise the Voisinage agreement so Irish vessels remain free to fish inshore waters around Northern Ireland.

The Government in Dublin had pledged to introduce legislation to give legal effect to Voisinage but has not two years on.

Mr Dodds, who demanded an explanation from the Taoiseach over the incident, said: “When Leo Varadkar talked about soldiers on the Border he didn’t mention the Irish sending warships with 76mm guns”.

“These heavy-handed tactics with our fishing vessels demonstrate that Leo Varadkar and Simon Coveney are entirely focused on Ireland and are fair weather friends to Northern Ireland,” he said.

The matter was raised in Westminster by Conservative MP Iain Duncan Smith, who said the detention had happened “without a huge amount of justification”.

DUP MP for Strangford Jim Shannon said the boats clearly were British fishing vessels. “They were illegally seized in waters that are disputed, waters that belong to this great nation,” he said.

Trawlermen representatives in Northern Ireland declined to comment on the arrests until the outcome of court hearings in Dublin, which they expected to take place on Friday.

Security sources said that UK-registered vessels, including those from the North, had previously been intercepted in a similar way, with one such incident occurring last year. The sources added that “sensitivities” related to Brexit had contributed to the level of publicity around the latest incident.

Some fishing sources in Northern Ireland said they suspected that Dublin was hoping to use the issue as a bargaining tool in any post Brexit fishing negotiations.

“There was a feeling that the Irish might use the court ruling as part of its negotiations over access arrangements to UK waters after Brexit,” said one.

Henry Reilly, an independent councillor for Newry, Mourne and Down, said he knew the boats’ owners, who had since returned home to Kilkeel, Co Down.

“I was told that they were very taken aback,” he said. “It’s a strange feeling whenever a big warship pulls up beside you and [BRINGS]you ashore.”

Fianna Fáil has dismissed as “ludicrous” claims that the move was linked to the ongoing Brexit negotiations.