Hi
The 1910 census shows Nicholas Kirkwood, 75, a farmer, living with his sister-in-law, Mary Gay, 60, in Contra Costa, Township 13, District 0175. He was born in Scotland and entered the USA in 1854. His parents were both Scottish so you need to search scotlandspeople for a Kirkwood family with children named Nicholas, Isabella, Agnes, Margaret and William, born about 1835.
Alternatively you can search for William's Australian death certificate, which I think will give the names of his parents.
Good luck
Gobbo