Thanks very much. I had no idea there was such detailed information in the 1900 census. That will make it much easier to look for a baptism record for William - searching for Irish records is notoriously difficult.
For their son Samuel, the age 21 must be correct - he would have been born in 1878 rather than 1868. He is aged 1 in the 1800 census transcription at www.familysearch.org The family were probably a bit careless with dates - in the 1880 census, Eliza is 3 years older than her husband.

William emigrating in 1857 aged 9 is fascinating - his mother, brother and sister stayed behind, all eventually moving to Scotland. But I have only found his father, Samuel Alexander Wray, mentioned in other people's records (he was dead by the time of the 1881 British census). He was a police sergeant, and I've had no luck finding him in Irish police records. It never occurred to me that he might have emigrated without his wife and two oldest children, but perhaps together with William. So maybe I should start looking for American police records.

Thanks very much.