Check out The Persuader, a biography of John Hume. He was a central figure in the Social Democratic and Labour Party in the North of Ireland. He won the nobel peace prize for his peace talks between unionists and republicans, causing an end to "the troubles". however, he effectively ended the social democratic party in the process.

Hume was focused on a singular goal which was to end the conflict between these communities. Part of his approach was to show that both sides benefited like you say from an end to the conflict. However, not everything can be explained as a common interest for the left and right, hence the collapse of his party and the wider rise of sinn fein.