Any list of surnames that claims to be complete is usually not. As with books of given names for babies, there are numerous combinations and ethnicities to be considered, as well as the machinations of creative minds.
For purposes of genealogy and family history, probably the most useful books are those based around ancestral countries. For example, Black's Surname of Scotland contains one of the best lists of surnames for that country. Most books of surnames give not only the current spelling, but may also give variations in spelling, origins of the surname, any affiliation with other names/clans/historical figures etc., and the locations where that particular surname is common. In the case of Scottish and Irish surnames, they may also give the Gaelic or Celtic equivalent.
Knowing your ancestor is from Ireland is good, but knowing where in Ireland is better, especially before 1864. Books listing surnames in Ireland have helped me locate possible counties where my Devitt ancestor lived before immigrating to America in the late 1790s. While many books have been written, there are an increasing number of web sites that will give you similar information with varying degrees of information. Genuki.org.uk/indexes/SurnameList is one of these useful sites. "Gen" stands for Genealogy; "uk" stands for the United Kingdom, and "I" stands for Ireland. Another site that includes a list of Irish surnames with Gaelic equivalent, meaning and coat of arms (where available) is http://www.ireland-information.com/heraldichall/irishsurnames.htm.
In the absence of or frustration in using online sources or books, and depending on the time period involved, researchers can use readily available lists of surnames such as those in national indexes of births, marriages, deaths, censuses, taxes etc. Sample a couple of years in various time periods to determine where people with your surname lived at the time. With any luck, you will be able to find out perhaps not only the county but also the parish or other geographic area where your ancestors may have lived.
From previous (and laborious) searching through indexes to early Tithe Applotments of Ireland, I had determined that the surname Devitt was common in western counties, particularly County Clare. Today we can use Broderbund's Indexes to Tithe Applotments and Griffith's Primary Valuation (land records) on CDs which will give you the same information in minutes. I learned through books that list surnames and give some information about the name that the surname Devitt is synonymous with McDermott or McDairmid. Mc means "son of" and O' means "grandson of". In early history, one name was sufficient for identification but, as the population increased, other means of identification became necessary and so relationship was used. Sometimes the place where that person was from or his occupation was used instead of relationship. In Ireland, family was important. We often associate Mc with Scottish surnames but the earliest Scots actually came from Ireland.
While you are doing your research, it may be helpful to create a list of surnames in your own family. You will be interested to see changes over time, including translations, spellings, occupations, national origins, Anglicization and relationships. And your list of surnames will also assist others who follow to access your research.