Cormac McQuinn: 'Brexit drives big wedge between Sinn Féin and long-time biggest UK supporter Jeremy Corbyn'
Rows over Brexit have divided political parties, friends and even families on the other side of the Irish Sea.
Now it seems to have driven a wedge between Sinn Féin and Labour leader Jeremy Corbyn, the man who has perhaps been the party's favourite British politician over more than three decades.
Mr Corbyn was one of the few who was willing to meet Sinn Féin figures such as Gerry Adams at the height of the IRA's murderous terror campaign in the 1980s and 1990s. He has spoken in the past of his support for a united Ireland.
However, Mr Corbyn, in an apparent bid to capitalise on Prime Minister Theresa May's difficulty getting her Brexit deal over the line is making overtures to Sinn Féin's arch-enemies, the DUP.
In recent days he suggested Labour could get an alternative Brexit deal. He told Sky News the DUP disliked the backstop designed to avoid a hard border in Ireland for "very good and sensible reasons".
Sinn Féin leader Mary Lou McDonald has been forced to distance her party from Mr Corbyn's stance on Brexit.
She said: "Jeremy Corbyn and all others who are in the British parliament are British politicians pursuing what they understand and believe to be British national interest."
Ms McDonald also said she has told Mr Corbyn: "We disagree and we differ dramatically on the issue of Brexit."
It's a far cry from remarks by Mr Adams earlier this year when he backed Mr Corbyn to be the next British prime minister.
Mr Adams called him an "outstanding politician", arguing Mr Corbyn "kept faith" and was open to talks on conflict resolution at a time when others weren't.
He was certainly willing to talk to Mr Adams at a time when the former Sinn Féin leader was public enemy number one in Britain.
Mr Corbyn caused outrage in the weeks after the Brighton bombing in 1984 by inviting Mr Adams and other members of Sinn Féin to the House of Commons. He was criticised by both Conservatives and his Labour colleague at the time. This was, after all, 10 years before the IRA ceasefire.
In 1985, Mr Corbyn opposed the Anglo-Irish Agreement saying it strengthened rather than weakened the Border.
Two years later, he observed a minute's silence after the SAS killed eight IRA members and a civilian during a Provisional IRA attack on a police base in Loughgall, Co Armagh.
Mr Corbyn was a supporter of the republican Troops Out movement during the Troubles, which campaigned for the British army to be withdrawn from Northern Ireland.
He was a regular visitor to the North during the Troubles and attended marches commemorating Bloody Sunday.
The relationship with Sinn Féin continued shortly before Mr Corbyn became Labour Party leader in 2015.
He met Mr Adams, the late Martin McGuinness and Ms McDonald in Westminster for a coffee with Mr Adams tweeting about the encounter.
Mr Adams was among the first to congratulate Mr Corbyn when he won the leadership race, saying on Twitter: "I have known Jeremy for many years... He is a good friend of Ireland and of the Irish peace process."
Critics of Mr Corbyn take a decidedly different view of his dealings with republicans over the years. He was heavily criticised on the issue during the UK general election last year.
DUP leader Arlene Foster denounced Mr Corbyn's alleged refusal to unequivocally condemn the IRA as "abhorrent". Former Northern Ireland secretary James Brokenshire accused him of having "IRA sympathies".
It came after Mr Corbyn said: "I condemn all the bombing by both the loyalists and the IRA."
Mr Corbyn later described the IRA's bombing campaign as "completely wrong" because it killed civilians. He denied he ever supported the IRA, and insisted in meetings with Sinn Féin members during the Troubles he always made the point there had to be dialogue and a peace process.
With his position on Brexit now at odds with Sinn Féin's, peace talks may now be needed between the long-time allies.