My Favorite County Mayo, Ireland Specific Websites:

County Mayo, Ireland Websites
Boomtown or Bust. The authors of this website have created a database documenting immigrants from Mayo to Canada in the 1800's -it has great unusual information that could open up a few trees. They also have a page on Mayo, Quebec... a town founded by Mayo immigrants, a vast array of information on Irish immigrants to Canada and a great resource list for the subject.
Family History in Ballycastle. This is a great website that focuses on the North East part of County Mayo. There are many wonderful pages and links on this site!
Connor's Genealogy Website. Extensive resource of County Mayo, Ireland records
County Mayo Gen Web Project. County Mayo resources, records, Civil Parish specific info
County Mayo Ireland Genealogy (UKI). Record resource and Civil Parish specific records.
County Mayo Library. Great resource for County Mayo Ordinance Survey, Estate Maps
Deesegenes: Pat Deese's Irish Roots Website. Extensive County Mayo Resources, records, links
East Mayo Genealogy
Irish Genealogy of Connacht. Connacht County specific records including County Mayo. A great website with information specific to Kiltimagh including the history of the Townland by Basil Burke who has written several great books on the area!
Mayo Books- If your ancestors came from County Mayo, be sure to check out this website. They have a vast array of books on everything County Mayo. I recently ordered three books from them- one on Brachloon School and District in Ballyhaunis (which had many great stories and a lot of historical information on the area- I enjoyed it immensely), Dear Old Ballina and Killasser- Heritage of a Mayo Parish.