March is officially designated as Irish-American Heritage Month in the United States, and on the 17th of the month each year people across the United States mark St. Patrick’s Day, a holiday with Christian roots that has morphed into a broader celebration of Irish culture.

Irish is the second-most frequently cited ancestry among US residents, behind only German, according to data from the US Census Bureau.

More than 33.5 million people nationwide claim Irish ancestry, which is more than seven times the current population of Ireland.

New England is the most Irish area, and Massachusetts is the most Irish state in the country, Census data show. Here are some highlights about Irishness in our region:

■ The percentage of people claiming Irish ancestry in Massachusetts is nearly 22 percent, the highest in the United States.

■ Irish is also the most popular of any ancestry claimed by Massachusetts residents, with more than 1.4 million people statewide claiming Irish ancestry. The next most popular ancestry in the state is Italian, with just over 900,000 people claiming that ancestry.

■ Marshfield is the most Irish municipality in the state. About 46 percent of the South Shore town’s 25,000-plus residents claim Irish ancestry, and it is near several other communities — including Hanover, Scituate, Norwell, and Abington — with high concentrations of people who claim Irish ancestry.

■ Yet people of Irish ancestry represent a slightly smaller share of the state’s population today than they did just several years ago. A similar trend is seen for Boston (where 14.3 percent claim Irish ancestry) and the nation.

Matt Rocheleau can be reached at matthew-.rocheleau@globe.com. Follow him on Twitter @mrochele.