Watson, most people have a “basic” alcoholic drink they enjoy when they’re not at a “real” bar…so as long as you have the usual (vodka, gin, rum, etc.) and some kind of mixer, like Coke or Sprite or juice, most people will be perfectly happy.
Having said that, don’t cheap out on the brands if you can possibly avoid it; I drink vodka, and I’m perfectly happy to drink it with a splash of Sprite, but if it’s cheap vodka I just won’t drink it at all. You don’t have to go “top shelf” but you’d do well to stock at least a good call brand. (Absolut/Skyy vodka, Bacardi rum, Dewar’s scotch, Tanqueray gin, Cuervo/Sauza tequila, etc…)
In addition to Bailey’s, you might want to stock a couple of liqueurs for people who don’t want what I call a “drink drink.” This would include Frangelico (hazelnut), Kahlua or Tia Maria (coffee), Amaretto (almond), etc., etc. They have a low alcoholic content and they’re nice on the rocks. Some of these make lovely mixers with vodka, too, or to spike an after-dinner coffee.
And if you want to have a little fun, DeKuyper’s Puckers make fun martinis and mixers, too; they come in flavors like watermelon, sour apple, peach, etc…and they’re inexpensive.
Glassware can be an expensive investment; as a “starter set” I would recommend short and tall rocks glasses, martini glasses, snifters, and red and white wine glasses. That should cover just about any drink a home bar would offer. (If you serve beer, a handful of chilled pints in your fridge or freezer are a nice touch.)
And the bartender in me can’t help but give you a word of caution: in Texas, the law holds you, as the host, just as liable for your guests as if you were a bar. So make sure you purchase jiggers and/or pour spouts for accurate/safe pouring, and that you know enough about liquors and their proofs so that you’re aware of how much alcohol people are actually drinking. (Some of the worst excesses I’ve ever seen are at parties where no one really knows much about what they’re serving/drinking/mixing.) If someone leaves your house really intoxicated and wraps their car around a tree, you’re legally liable, so always err on the side of caution if you’re throwing a party, and always offer food or snacks.
And there ends the Safety and Liability Lecture. 