Every year in the runup to St. Patrick’s Day, the Census Bureau releases a demographic profile of Irish-Americans. For anyone familiar with the arduous history of the Irish in this country, the progress report is an annual reminder of America’s ability to assimilate newcomers in search of a better life.

It was the potato famine that began driving large numbers of Irish to leave home in the late 1840s. This migration, along with mass starvation and disease, would eventually cost Ireland around a third of its population. Some went to Great Britain, but the overwhelming majority came to America. Today the number of Americans of Irish descent (32.3 million) is nearly seven times as large as the population of Ireland (4.7 million).

The peasants fleeing Ireland had a shorter life expectancy than slaves in the U.S., many of whom enjoyed healthier diets and better living quarters. Most slaves slept on mattresses, while most poor Irish peasants slept on piles of straw. The black scholar W.E.B. Du Bois wrote that freed slaves were poor by American standards, “but not as poor as the Irish peasants.”

The Irish who left for America were packed into the unused cargo space of wind-driven ships returning to the U.S., and the voyage could take up to three months, depending on weather. These cargo holds weren’t intended to carry passengers, and the lack of proper ventilation and sanitation meant that outbreaks of typhus, cholera and other fatal diseases were common. Emigrants slept on 3-by-6-foot shelves, which one observer described as “still reeking from the ineradicable stench left by the emigrants of the last voyage.”

In 1847, 19% of the Irish emigrants died on their way to the U.S. or shortly after arriving. By comparison, the average mortality rate on British slave ships of the period was 9%. Slave-owners had an economic incentive to keep slaves alive. No one had such an interest in the Irish. ...