While millions of Americans celebrate St. Patrick's Day on Friday, the actual number of citizens who identify themselves as "Irish" is dwindling. According to Census data, there are 32.7 million Americans, who claim to be of Irish descent including 3 million who say they are of Scotch-Irish heritage.

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In 1990, 38.7 million Americans, representing 15.6 percent of the total U.S. population, claimed Irish ancestry, while 5.6 million identified themselves as Scotch-Irish, according to Pew Research Center. In 2013, the median age of those claiming Irish heritage was 40.5, while the median age of those claiming Scotch-Irish ancestry was 52.1. The median age of the entire U.S. population is 37.8.

Many Irish immigrants arrived in the U.S. in the 1840s and 1850s, while the Scotch-Irish emigrated at the beginning of the 1700s.

“Irish Americans are at least 5 percent of the population in most counties across the U.S., and 10 percent or more in most of New England, New York state, New Jersey, eastern Pennsylvania, and other smaller counties across the country,” Jed Kulko, chief economist for Trulia told Irish Central last month.

Massachusetts has the highest Irish-American concentration, with 21.6 percent of residents, followed by New Hampshire with 21 percent. Rhode Island is third at 18.3 percent and is followed by Vermont (17.9 percent) and Maine (17.6 percent.)

Irish Americans are the second-largest ancestry group in the U.S. after Germans, according to Pew Research Center.