What makes it Irish Whiskey (other than it's from Ireland)?

Irish whiskey is made from malted barley (or a combination of malted and unmalted barley) and other cereal grains such as wheat, rye, and oats.  Unlike Scotch, however, the malt is dried in a kiln which has a solid floor, thus disallowing the smoke from whatever fuel is used to come in contact with the grain.  This eliminates the "smoky" taste you will find in Scotch and allows Irish whiskey to be smoother, if a bit less distinct of a whiskey.  It is a whiskey that should appeal to both bourbon and Scotch drinkers alike, although its popularity in the United States is limited (except around St. Patrick's Day when we all fancy ourselves Irish and use it as an excuse to drink).

What Irish whiskey should I buy?

Unlike all other whiskies, there are really no "bottom-of-the-barrel" Irish whiskies (at least in my corner of the world).  Even the most affordable Irish pushes $20 for the fifth and is supremely drinkable.  But, there are super-premium Irish whiskies, too.  So, I've eliminated the Budget Model category I have for other liquor lists in my list of Irish whiskey.  I'll just keep it at Ultimate, Top Shelf, and Middle-Class (see right column).

That about does it for Irish Whiskey.  It's not my favorite whiskey to drink, but most of them are smooth and enjoyable without giving the smoky jolt you'll get from Scotch (especially Scotch from the island of Islay).

Also see these sections of BlueKitchen.net
The Best Bourbons
The Best Canadian Whisky
The Whiskey Glossary

As always, enjoy!
Ultimate
Jameson 15-year old Pot-Stilled  ($110)
Bushmills Malt 21-year  (n/a)
Bushmills Malt 16-year  ($63)
RedBreast  ($45)
Knappogue  ($35)

Top-Shelf
Jameson 12-year-old  (a.k.a. Jameson 1780)  [$35]
Jameson  ($24)
Bushmills Malt 10-year  ($33)
Black Bush  ($35)
Tullamore Dew 12-year-old  ($36)

Middle-Class
Tullamore Dew  ($23)
John Power & Son  ($18)
Irish
Whiskey