If you're lucky enough to know where your Irish ancestor(s) originated, you'll be motivated to learn more about the location - where it is and what it means. There are a number of good resources when it comes to Irish place names. Many Irish place names are "Anglicized" or phonetic spellings of the word in the Irish language.  As an example, "Bally" comes from the Irish word "Baile", meaning town or townland.  Studying Irish place names unveils Irish geography and history, as well as the development of the Irish language. 

Here are two excellent websites for researching Irish place names:

http://www.logainm.ie/ - Placenames Database of Ireland.  (First, make sure you're looking at the English version by clicking in the upper right corner of the screen.)  You can search place names here in two ways, by interactive "map" or by "data", which shows archival records about the meaning of the word.  My husband's Hickey ancestors came from the townland of Cahernahallia, in County Tipperary.  The "data" feature on this site shows there have been many spelling variations of Cahernahallia over the centuries. The meaning of the place name is related to its downward slope or "cliff".  Using the "map" feature, I can zoom in and look at the topography - and see the place names of a half-dozen other townlands within a mile or so of Cahernahallia.  

http://www.irish-place-names.com/ - Villages, Towns and Townlands in Ireland.  Initially, I couldn't query this site using my browser (Internet Explorer), but apparently the problem has now been fixed.    The table shows 60,000+ place names in alphabetical order, but you can limit the search by typing in part of the town/townland name or by sorting on one of the columns (e.g., the county or civil parish).  This search vehicle might be particularly valuable if you weren't certain of a precise place name or its spelling.  However, the site does not appear to have a complete listing for Northern Ireland.  Here's the description of what locations are included:  "The Irish Place Names database is based on the Index to the Townlands, and Towns, Parishes and Baronies of Ireland published in 1861 after the Census of Ireland carried out in 1851. This database lists all of the villages, towns and townlands that were enumerated, as well as the Barony, Civil Parish and Poor Law Union that each was located in".