Previously, it had been thought that the group, which died within weeks of starting gruelling work on the Philadelphia and Columbia railroad in 1832, were cholera victims.
Two skulls unearthed at a probable mass grave near Philadelphia last week showed signs of violence, including a possible bullet hole. Another pair of skulls found earlier at the site also displayed traumas, seeming to confirm the suspicions of two historians leading the archaeological dig.
“This was much more than a cholera epidemic,” William Watson said.
Dr Watson, chairman of the history department at nearby Immaculata University, and his twin brother Frank have been working for nearly a decade to unravel the mystery.
Anti-Irish sentiment made 19th-century America a hostile place for the workers, who lived amid wilderness in a shanty near the railroad tracks. The land is now preserved open space behind suburban homes in Malvern, over 30km west of Philadelphia.
The Watsons and their research team have recovered seven sets of remains since digging up the first shin bone in March 2009.
The Watsons believe some immigrants were killed by vigilantes because of anti- Irish prejudice, tension between affluent residents and poor transient workers, or intense fear of cholera – or a combination of all three.
Their theory is supported by the four recovered skulls, which indicate the men probably sustained blows to the head. At least one may have been shot, said Janet Monge, an anthropologist working on the project.
“Violence was the cause of death and not cholera, although these men might have had cholera,” Monge said last week.
The Watsons learned in 2002 of the workers’ demise from the personal papers of their late grandfather, who worked for the railroad long after the men died. Their quest, called The Duffy’s Cut Project, is named after Philip Duffy, who hired the Irishmen to build a section of railroad known as a cut.
When the immigrants died in August 1832, Duffy ordered his blacksmith to burn the shanty for sanitary reasons and bury the bodies in the railroad fill, the Watsons say. The families were never told of their deaths.
A passenger list for the John Stamp, a ship that sailed from Ireland to Philadelphia four months earlier, offers possible identities for 15 workers who came from Donegal, Tyrone and Derry.
Early on, the Watsons tentatively identified one victim as 18-year-old John Ruddy. They have since found a section of teeth with a rare genetic anomaly, a missing upper molar, shared by some Ruddy family members in Ireland. Researchers hope for DNA confirmation in about six months.
Michael Collins, Ireland’s ambassador to the US, visited Duffy’s Cut last summer and said it was an important story to tell.