HI Loretta,
Do you know much about your great-grandfather's emmigration? The dates, reasons why he left, who he may have travelled with etc..? Do you know his port of arrival (e.g. Castle Garden, New York...etc.), often more information was given at the point of arrival than the port of departure. If you haven't already done so you can try searching for him at:
http://www.castlegarden.org/ or http://www.archives.gov/research/immigration/passenger-arrival.html
You could also try posting as much information as you can concerning your relatives. i.e. dates of births/marriages/deaths, names of parents/siblings etc.. The more information you post, the more likely it is that one of our volunteers will be able to assist you.
Some other sites which may be of use to you are:
http://www.dippam.ac.uk/ (Documenting Ireland...)
http://www.nationalarchives.ie/genealogy1/genealogy-records/parish-records-marriage-licences/ National Archives of Ireland
http://www.corkarchives.ie/genealogy/ Cork Archives
http://www.irishtimes.com/ancestor/browse/ Irish Times 'Browse' the records for Cork..
You could try checking Griffith's Valuation (1848-64). Griffith's is freely available here: www.askaboutireland.com or here: www.failteromhat.com Failte Romhat has lots of other useful links you could try looking at. Also https://familysearch.org/ is a free site that can be very helpful.
You may also try to contact: http://www.cobhheritage.com/ Cobh Heritage Centre for some assistance.
I wish you the best of luck with your search.
Kind regards,
Genealogy support
