Origin of I'll Take you home Kathleen

 The Song I'll take you home Kathleen is a popular song, a lament about promising to take his homesick wife back to Ireland.

So a traditional Irish ballad, right?

Nope. according to Wikipedia it was written by an American of German ethnicity, Thomas Paine Westendorf,

He wrote it for his wife (who was, however, named Jane), who had made a visit to her home state of New York due to homesickness. It is an answer song to a popular ballad of the time, "Barney, Take Me Home Again," composed by Westendorf’s close friend, George W. Brown

so why is it considered an Irish song? 

from LiveAbout.com:

Despite its origins, Westendorf struck a romantic chord in the hearts of some Irish music-lovers with his use of the popular Irish feminine name "Kathleen" as well as his use of poetic Irish-ish (for lack of a better term) language, all describing the wistful thoughts of home shared by so many immigrants, and the song quickly entered the repertoire of Irish folk singers on both sides of the Atlantic, as well as plenty of pop singers, including Elvis Presley.