First found in county Mayo, where they held a family seat from ancient times.

A great wave of Irish migration occurred during the 19th century as a direct result of English colonial rule and tight-fisted absentee landlords. Many of these Irish immigrants boarded passenger ships bound for North America. Those who migrated early enough were given land in either British North America or the United States; those who came in the late 19th century were typically employed in industrial centers as laborers. At whatever age they undertook the dangerous passage to North America, those Irish immigrants were essential to the speedy development of the two infant nations to which they arrived, whether they broke and settled land, helped build canals, bridges, and railroads, or produced products for consumer consumption. An examination of immigration and passenger lists has uncovered a large number of immigrants bearing the name Killen or one of its variants: Jenkins Killen, who settled in Virginia in 1805; Alexander, Denis, James, John, Patrick, and Robert Killen, who all arrived in Philadelphia between 1840 and 1860.

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