You might need to recheck the census details as the 1920 census shows Isabel as having been born in Oregon.
It seems that the Isabel Hughes was born in Oregon approx 1886. She is in the 1920 Census in the home of her widowed mother Kathleen Frances Hughes.
US 1920 Census - Oregon Multnomah
Kathleen Hughes Head Widow? 68
Isabel Whitfield Daur Married 34
Helene Hughes Daur Single 33
William Whitfield Gson Single 1 4/12
Emily Whitfield Boarder Single 35
Kathleen indicates she arrived in 1880 and Emily 1912
US 1930 Census - Oregon Multnomah
Kathleen Hughes Head Widow 78
Eva Smith Sister Single 72
Rhoda Smith Sister Single 68
All of them indicate that they and both parents were born in the Irish Free State.
I have also just found a Familysearch.org reference for Isabel's birth.
"Oregon, Births and Christenings, 1868-1929," index, FamilySearch (https://familysearch.org/ark:/61903/1:1:FZ5F-HXL : accessed 7 June 2015), Wm Hughes in entry for Isabel Hughes, 11 Nov 1885; citing Heppner, Morrow, Oregon, reference rn DF 412; FHL microfilm 2,229,469.