Apologies - I realise this is probably not relevant but I have accessed the 1861 census for Greenock on Scotlands People. And I will list the detail in case at some point it means something to some one
It has Mary McDonald, born Barrhead and I am sure it says Mariner's wife. Her entry is at the top of the page but a quick search of Donald's does return an entry on the same GROS fields.
The it has Margaret aged 11, but Duncan is listed as aged 7, then there is a william aged 4, alexander aged 3, then an annie (not sure age).
Still no sign of a Hugh