Thank you for looking into this mystery, I certainly appreciate your time spent on this.
When talking about George I believe he married late in life or I'm sure his obit would have stated his first marriage. In his obit it states that they came to America in 1869 to Prairie Creek township, Dubuque County, Iowa.
I also have a declaration on intent for U.S. citizenship that states he arrived in June of 1867. So my thoughts are that he arrived with his family in 1867 and moved to Prairie Creek in 1869 or they may have come directly to this area in 1869?
In the 1870 U.S. census it shows them not owning land and having 200 worth in personal estate (not much compared to the neighbors).
I do know that George and Mary were Roman Catholic. George's occupation was a farmer/ laborer. Your right with John Graham at Michael Rea's working as a servant, his name is also under his family's of that same year. John's sister Ellen also shows working as a servant for McCollins.
George's family looks to have been very poor which indicates why he sent his kids out to work. I know George did not own land in Iowa, but his son William bought there home which George retired in until his death in 1900.
With that being said in Ireland I'm sure they were also poor and traveled by cheapest means and George probably didn't own land there either.
If there is anything else that might aid you, I would be more than happy to help.