Hi There
The oldest records you can search online are the TITHE APPLOTMENTS (1823-37)
http://titheapplotmentbooks.nationalarchives.ie/search/tab/results.jsp?s...
(Search by name or Browse by townland)
Then there are the Griffiths Valuations (1850's), where you can locate the townland of origin.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=doName...
More recent records are the 1901 and 1911 Census
http://www.census.nationalarchives.ie/search/
These are the Irish Records available at Familysearch.org
https://familysearch.org/search/collection/list#page=1&countryId=1927084
You could also Browse through these Sites with Mayo Gravestones
Cong Cemetery on Findagrave -
http://www.findagrave.com/cgi-bin/fg.cgi?page=cr&GSvcid=244803&CRid=2267...
http://www.irishgraveyards.ie/search_page.php
Some townlands to consider near Cong would be Clonbur, Cross, Ballinrobe, The Neale. (East of Cong as you mention would be Cross, Shrule, Headford, )
http://www.irelands-directory.com/Mayo/Cong/maps.html
I come from West Mayo, but someone else maybe able to supply more information about the local area.
Good luck with your research, feel free to ask/email me if you have any difficulty with the above sites
Sl?inte
Dympna