The centenary of a Belfast strike when Catholic and Protestant workers united briefly has been marked in Dublin.
There are statues to Larkin in Belfast and Dublin
More than 5,000 dockers downed tools for four months in the 1907 Belfast Lockout for better pay and conditions.
Royal Irish Constabulary police later mutinied when ordered to escort "scab" workers to the docks.
Soldiers were called in to end the strike, which was led by trade union crusader Jim Larkin who is commemorated with statues in Belfast and Dublin.
Union leaders from Northern Ireland and the Republic gathered on Tuesday at Larkin's grave in Dublin's Glasnevin Cemetery to lay a wreath to mark the centenary.
Catholics and Protestants were just as divided by politics and religion in those days, but Larkin achieved a fragile unity for several months as Falls Road and Shankill Road came together, said political historian Eamon Phoenix.
"Larkin was a giant of a man and he used his charisma and oratorical skills to articulate grievances of the working classes," he said.
"Sectarianism was sidelined and Home Rule dropped off the agenda for a short period in 1907."
A Siptu spokesman said of the commemoration: "This is the first of a series of events to mark the 1907 general strike in Belfast, when Catholic and Protestant workers united to demand trade union recognition and decent working conditions.
"The strike brought the city to a standstill, even the police joining in the dispute. It was eventually put down by the use of troops."
Larkin later founded the unions that eventually formed Siptu and also co-founded the Irish Labour Party with James Connolly in 1912.
Former Irish Labour Party leader, Ruairi Quinn, who attended the event, said the Belfast Lockout was one of the first examples of the worker radicalism and led to the famous Dublin Lockout in 1913.
"The Belfast strike was a major event in the early years of the trade union movement," Mr Quinn added.
Casual workers such as dockers and carters in Belfast at the turn of the century often worked under extreme hardship. They received no holidays and were often laid off during work shortages.
Most of the striking workers were employed by industrialist Thomas Gallagher who owned the Belfast Steamship Company and members of the National Union of Dock Labourers.