Ex-Sinn Fein staffer joins Jeremy Corbyn's leadership team
In 1984 - a decade before the first IRA ceasefire - Mr Corbyn met with Mr Adams in London, and a year later he opposed the Anglo-Irish Agreement saying it strengthened rather than weakened the border.
He also appointed John McDonnell as shadow chancellor, who has apologised for saying in 2003 that IRA members should be honoured for bringing the British to the negotiating table.
Ms Fisher first joined Labour at the age of 18, a source said, and is an active member of the party's Islington South branch.
She took part in arrangements for the Labour women's conference in the 1980s and 1990s and was involved with the Labour Party Irish Society from when it was established in 1998.
She also took part in a cross-party initiative called Friends of the Good Friday Agreement, which Labour former shadow Northern Ireland secretary Kevin McNamara was involved in launching.
Ms Fisher then worked for the Sinn Fein Parliamentary Group, which the source said was set up to help the Northern Irish party do political outreach work in Westminster during the peace process.
A Labour Party spokesman said: "We don't comment on staffing matters."
One Labour MP told the Press Association the move was "unsurprising" as Mr Corbyn and Ms Fisher are friends, but "completely ill-judged".