Hi
According to the 1920 US census, Thomas and Abigail arrived in the US in 1879, and are both naturalized citizens.
In 1910 and 1920, they were living in Fergus county.
Gen web says that Fergus County was formed in 1885/6. If searching prior to that date, look at Meagher and Chouteau county records.
Regards,
Valerie