Alcohol and morphine function (as drugs often do in literature) as symbols of retreat. Basically, no one in the family has anywhere to go – literally or metaphorically – so they have two options: fight or flight. They fight often (especially the male characters), but they also spend a whole lot of time fleeing, turning to drugs and alcohol to hide from reality. We talk about Mary's particular dependence on morphine in her "Character Analysis," so let's get specific with what the Tyrone men are up to.
The Tyrones don't just drink any alcohol; they drink bonded bourbon. Bonded means the bourbon is really good (aged four years and distilled by one brewer for a season at a distillery) and, thus, more expensive. This is some seriously high quality bourbon, and it's another hint – along with all those real estate deals – that James is willing to spend extra money, so long as he is the primary beneficiary. It's also a social class symbol – poor people don't drink bonded bourbon.
Bourbon is also an important choice because bourbon is basically the American alcohol. This stuff is classic Americana, a whiskey made from corn and named after the county in Kentucky where it was invented.
So here's our question: why Jim Beam instead of Jameson's? Let's not forget that James is all about Irish patriotism, yet he doesn't drink Irish whiskey. While there may be a significant difference in flavor between bourbon and Irish whiskey, Ireland obviously has its own illustrious history of whiskey brewing. We don't want to push this too hard, because there may be issues of price and availability, but with all of his posturing about his roots and his defense of all things Irish, it's surprising that James only ever drinks the American stuff.
Does this have any symbolic meaning? Looks like it to us. James has "made it," has assimilated successfully into American culture. He's a representative of the American dream, and, just as he's ditched his childhood of impoverishment and labor, he's ditched the liquid representative of his abandoned culture – Irish whiskey.