Hi,
Thank you very much for your message.
In order to find her parents/grandparents you will have to look in church records as civil registration did not begin until 1864. Prior to this all births, deaths and marriages were recorded by the local parish church. As a result you really need to know the parish that the event took place in to know where you should be looking for records. Do you know which parish in Sligo Catherine was born in?
Have you found her marriage certificate? If you can find this it may give you some clues as to where she was from. For more information on marriage certificates and where to find them follow this link:
http://www.irish-genealogy-toolkit.com/Irish-marriage-records.html#.UbWwGEBQEaI
You could also try searching for her birth record on such sites as www.familysearch.org or www.rootsireland.ie (please note this website charges)
Once you know where Catherine was born more avenues of research will open up to you. In the meantime here are some other websites that you may find useful:
The National Archives of Ireland http://www.nationalarchives.ie/genealogy1/genealogy-records/introduction/
The National Library of Ireland http://www.nli.ie/en/family-history-introduction.aspx
Genealogy Links: http://www.genealogylinks.net/uk/ireland/sligo/index.html
Irish Times: http://www.irishtimes.com/ancestor/index.htm
Sligo Library: http://www.sligolibrary.ie/sligolibrarynew/localstudies/
Kind regards,
Genealogy Support