Dear Irene
As you can see from the link below, Castlerea is in the Civil Parish of Kilkeevin:
http://www.irishtimes.com/ancestor/fuses/townlands/index.cfm?fuseaction=TownlandsInCivil&civilparishid=2055&civilparish=Kilkeevin&citycounty=Roscommon
Records for the corresponding Roman Catholic Parish Roscommon begin in 1837. There are some places where you can search online, however these websites do not have complete collections and many do not cover the period after 1900:
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.
http://churchrecords.irishgenealogy.ie/churchrecords/ have begun a project to upload Irish church records to their site free of charge. However so far only Counties Dublin, Kerry, Carlow and parts of Cork have been covered.
Some other websites that may be helpful are:
The National Archives of Ireland: http://www.nationalarchives.ie/genealogy1/genealogy-records/introduction/ - you can search the 1901/1911 Census on the National Archives page too
The National Library of Ireland: http://www.nli.ie/en/family-history-introduction.aspx
The National Archives UK ? genealogy search:http://www.nationalarchives.gov.uk/records/looking-for-person/
The Public Records Office of Northern Ireland: http://www.proni.gov.uk/index/family_history.htm
Irish Times: http://www.irishtimes.com/ancestor/index.htm
Irish Genealogy Tool kit: http://www.irish-genealogy-toolkit.com/
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.
Best of luck with your search
Clare Doyle
Genealogy Support
