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 178-year-old 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, about 20 miles 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, following years of fruitlessly scouring the area for the men's final resting place. One victim has been tentatively identified, pending DNA tests.
The brothers have long hypothesized that many of the workers succumbed to cholera, a bacterial infection spread by contaminated water or food. The disease was rampant at the time, and had a typical mortality rate of 40 percent to 60 percent.
The other immigrants, they surmise, 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.
Now, their theory is supported by the four recovered skulls, which indicate the men probably suffered blows to the head. At least one may have been shot, said Janet Monge, an anthropologist working on the project.
"I don't think we need to be so hesitant in coming to the conclusion now that violence was the cause of death and not cholera, although these men might have had cholera in addition," Monge said.