In the 1820s, Boston's population grew rapidly, and the city's ethnic composition changed dramatically with the first wave of European immigrants. Irish immigrants dominated the first wave of newcomers during this period, especially following the Irish Potato Famine; by 1850, about 35,000 Irish lived in Boston. In the latter half of the 19th century, the city saw increasing numbers of Irish, Germans, Lebanese, Syrians, French Canadians, and Russian and Polish Jews settled in the city. By the end of the 19th century, Boston's core neighborhoods had become enclaves of ethnically distinct immigrants—Italians inhabited the North End, Irish dominated South Boston and Charlestown, and Russian Jews lived in the West End. Irish and Italian immigrants brought with them Roman Catholicism. Currently, Catholics make up Boston's largest religious community, and since the early 20th century, the Irish have played a major role in Boston politics—prominent figures include the Kennedys, Tip O'Neill, and John F. Fitzgerald.

How did Boston's population change in the 1820's?