On FS there is a record of a George Killen who left Ireland, emigrated to the US on 13/11/1855 on the Tonawanda.
The intriguing thing about this is that he appears to have been 'accompanied by Thomas' as the record says. Port of entry is Philadelphia.
Then there is an entry for George, Elizabeth and William Killen (baby) in 1860 census Sciota Township, Pickaway, Ohio, United States
NB the father of the George on Clogher marriage is William
In 1870 a George is in Wagner Clayton Co Iowa. he has Elizabeth, 8 year old Thomas born Ohio and 2 year old Emma born Iowa. He is a farmer and has real estate valued at $800.
1880 a George & Elizabeth appear in Dougherty, Cerro Gordo, Iowa, United States
1905 he completes Iowa state census 1905. He is widower, been in US 50 years and in Iowa for 40 years.
Not sure if this is your guy?