Hi Joe,
One of your Kiwi connections here. Don't know what info I last passed on to you but found a marriage record for Elizabeth Courtney to Thomas Darby on 6 Dec 1824 in Coalisland. It turns out the name given for William Darby's death certificate was a mistake i.e. father being William when it should have been Thomas.
Thomas is also confirmed in the 1841 census for the family. Thomas, Elizabeth and William are all living in Whitburn, Lothian (I think from memory). A second son, John, was born c1844 in Glasgow. He married twice and had 10 children. Unfortunately a large number of them died in infancy. John, himself, died from laudanum poisoining in 1888.
Can provide you with updated info on what I've found if you want to send me an e-mail.
Regards,
Jodie.