Hi
Thank you for your message.
You can check for information about the frequency of the O'Neil name in the mid-19th century and any other variant spellings of the name here: http://www.irishtimes.com/ancestor/surname/
If you haven't already - you could try checking the land records called the Tithe Applotment Books (1823-38) http://titheapplotmentbooks.nationalarchives.ie/search/tab/home.jsp or the later Griffith's Valuation (1848-64) http://www.askaboutireland.ie/griffith-valuation/ or the Church of Latter Day Saints (LDS) https://familysearch.org/
Failte Romhat has lots of other useful links you could try looking at ). www.failteromhat.com
Some additional sites/resources which might be of use to you include:
The National Archives of Ireland http://www.nationalarchives.ie/genealogy1/genealogy-records/introduction/
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 Mayo County Library: http://www.mayolibrary.ie/en/Genealogy/
Remember to post as much information as you can with regard to the people you are researching. 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.
Kind regards,
Genealogy Support
