Centre for Migration Studies - Ulster American Folkpark. Has a database of migration documents that can be accessed on site.
Irish Family History Foundation - Will do fee searches. The Wicklow Parish records may now be available for search.
(Mac)Geoghegan Clan - Includes forum
Geoghegan History - Broad history of the family from Niall, high king of Ireland
Geoghegan Surname - All the Gahagan variations
Geoghegan Family Genealogy - Kentucky Geoghegans
Donegal Ancestry - Message board. I posted a query for Thomas Gahagan there.
Ireland Surnames - Links to family web pages
Doire.org - Donegal family records and research tips
University College Cork Migration Resources - works with the Centre for Migration Studies in Ulster
National Archives of Ireland - Has a fee based genealogy service