“This is the challenge that we have, that we have advocated for an open border on this island,” she said. “It is absolutely a challenge.”

“I would say it’s higher than 80 per cent,” the minister added as she gave evidence on Ireland’s decision to opt into new EU asylum rules that aim to speed up migrant returns.

The UK and Ireland share a Common Travel Area, which predates both countries’ membership of the EU.

Infrastructure on the border between the Republic and Northern Ireland has gradually been dismantled since the Good Friday Agreement more than 25 years ago.

During the Brexit negotiations, Dublin insisted that the border on the island of Ireland be kept “invisible”, without infrastructure such as checkpoints.

It argued this would protect the peace process and safeguard Ireland’s place in the EU’s Single Market. Brexiteers accused Dublin of “weaponising” the border.

The UK and EU eventually agreed to create an Irish Sea border for British goods and animals entering Northern Ireland to prevent a hard Irish border.

The UK region continues to follow hundreds of EU rules as part of the agreement, which enraged some Unionists.