South Lanarkshire Unison branch secretary Stephen Smellie told the Morning Star: "New Lanark is one of the roots of our trade union, co-operative and socialist movement."
The New Lanark mills were bought by utopian socialist Robert Owen in 1779 and he started to run the site on philanthropic and co-operative principles.
Mr Smellie said that the event helps build awareness "of our collective history so we can better face the serious challenge of today."
Northern Ireland regional secretary Patricia McKeown addressed the gathering in one of the historic mill buildings.
"So much of what was fought for in these centuries has been under attack for the last 30 years or so," she said.
"We must never forget how incredibly powerful we can be as ordinary working people.
"Everything our forebears fought for must be something we are prepared to fight for… no matter what they throw at us."
There was music from Carol Laula, Alastair McDonald and the Co-operative Funeralcare Brass Band, as well as song-writing workshops and falconry and the obligatory bouncy castle and facepainting.