Hi Anthea
Let me know if you would like to meet up while you are in Tuam. I live just outside the town and while I will be in the UK for a few days in April would love to help you out if I can.
Let me know at cdoyle@irelandxo.com your intended visiting dates.
There is a Church baptism for a Josephine baptised on 22-Apr-1855 in Tuam to a father John Costello and a mother Anne Brook. Other children include: Elizabeth: 21-Mar-1850, Frances:18-Mar-1847. I can't see a marriage record for the couple in Galway. You can see the originals as Elwyn suggests above on the NLI website.
There are no Brook families listed on the Griffith's Valuation in the mid-1800s so it may be the case that Ann was not a local girl:
http://www.irishtimes.com/ancestor/surname/index.cfm?Surname=Brook&fuse….
There is a John Costello entry on the Griffith;'s Valuation for Townparks, Tuam and this COULD be your John:
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=doNam…
Let me know what you think.
Best wishes
Clare Doyle
Genealogy Support