New documents show former British Prime Minister Margaret Thatcher admitted the UK 'got it wrong' in relation to the border between the Republic of Ireland and Northern Ireland.

The State papers released under the 30-year rule also show Mrs Thatcher was 'depressed' over the situation in the North in the 1980s, amid concerns about violence.

She confided in then-Taoiseach Garrett Fitzgerald that the UK 'got it wrong' in 1921 with the border.

State papers and records from 1985, the year the Anglo-Irish Agreement was signed | Image: RollingNews.ie

1985 saw Mrs Thatcher and Mr Fitzgerald sign the Anglo-Irish Agreement - one of the first official documents to acknowledge the minority wish in Northern Ireland for a united Ireland.

The Agreement also granted the Irish government an advisory role in the North.

It was voted in by the largest parliamentary majority Mrs Thatcher enjoyed during her three terms as British prime minister.

Dr Garret Fitzgerald and UK Prime Minister Margaret Thatcher shake hands after signing the Anglo Irish Agreement in Hillsborough outside Belfast in 1985. In the centre is Northern Ireland Secretary of State Tom King | Image: RollingNews.ie

While it was one of the first steps in the process towards the Good Friday Agreement, it was largely rejected by both Unionists and Republicans.

Ralph Riegel is southern correspondent with the Irish Independent.

He told Newstalk Breakfast many of the discussions between the two leaders were about the developing situation in the North.

"Dr Fitzgerald is praising the RUC, saying they're working much better with the gardaí, there's a greater pooling of information, we're doing better - but it's impossible to secure that border.

"And Margaret Thatcher basically stops and says 'Look, we got it wrong in 1921' - which I think is the first admission of just how artificial the border that was created back in the '20s actually is".