The attack took place on 5 January 1976, just after 17:30 GMT.
A red Ford Transit bus was carrying the men home from their workplace in Glenanne, along the rural road to Bessbrook.
As the bus cleared the rise of a hill, it was stopped by a man standing in the road flashing a torch.
As the vehicle came to a halt, 11 other men, all masked and armed, emerged from hedges around the road.
The IRA men ordered the passengers out of the bus demanding to know the religion of each of the men.
One of the workers, who identified himself as a Catholic, was told to leave.
The gang then opened fire on the remaining passengers, killing 10 Protestant workmen and seriously wounding another.
No-one has ever been held to account for the murders.