Lesley,
Civil registration of BMD only began in 1864, so you won't be able to get a copy of Amelia's marriage cert. The Mallow Heritage Centre would be able to help you, but they are rather pricey. You could try writing to the parish priest of KIlbrogan (with a suitable donation!) and ask him for the details of the marriage.
I presume you've seen the HORNIBROOKs on the IGI? There are 3 listed for the parish of Kilbrogan, including the marriage date of your Amelia.
Also, Griffiths Valuation at
http://www.failteromhat.com/griffiths.phpshows 8 HORNIBROOK heads of household in Kilbrogan, surely relatives of Amelia?
Good luck
Leofric