Hi Joanna,
Thanks for your message. Unfortunately, it may be difficult to find information without a more specific place of origin. Most Irish record sources- church records, land records, census records- are based on specific locations. Until you know a specific location within Ireland, preferably a parish or placename, it may be difficult to do anything more with these records.
Do you have any documentation pertaining to the family that mentions any place names in Galway at all? This can include wills, marriage/death records,census records, letters, diary entries etc.
There are some places where you can search online, however these websites do not have complete collections:
www.familysearch.org has a huge database of genealogical records including some church records for Ireland.
www.rootsireland.ie have a large collection of Irish Church records, however you have to pay to use this site.
Here is a list of Casey?s that were recorded in Galway in the Tithe Applotment Books. There were no Marks recorded and the records do not have any genealogical information anyways so it will be difficult to know if any are relevant to you. However it will give you an idea of what parishes there were Casey?s living in around the time of their emigration.
http://titheapplotmentbooks.nationalarchives.ie/search/tab/results.jsp?surname=casey&firstname=&county=Galway&townland=&parish=&search=Search&sort=year&pageSize=10
Some other websites that may be useful are:
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
Irish Times: http://www.irishtimes.com/ancestor/index.htm
Irish Genealogy Tool kit: http://www.irish-genealogy-toolkit.com/
Genealogy Links: http://www.genealogylinks.net/uk/ireland/galway/index.html
From Ireland: www.from-ireland.net
Remember to post any new information that you find here. The more information you post, the more likely it is that one of our volunteers will be able to advise or assist you. Also include information concerning which sources you may have already used so others may further your search.
Kind regards,
Genealogy Support