The ironic t-shirt is a t-shirt that is somehow meant to communicate irony. More importantly, the ironic t-shirt is a stereotypical garment associated with the "hipster", or more importantly, with a certain subset of hipsters. There are actually two subsets of ironic t-shirt, which will be explained as I explain the history of both t-shirts and hipsters.
T-shirts are mass produced in the United States, as well as across much of the world. The t-shirt is a fairly generic, unisex and widespread option for covering the torso, either alone or with something else over it. There is nothing that special about t-shirts. And because of this, t-shirts are often very cheap, and are often given away or sold cheaply to commemorate special occasions and places. A person on vacation might pick some up as mementos, or get them for attending an event. Often, these t-shirts lose their meaning quickly and are given away to a second hand store. Thus, any second-hand store is usually filled to the brim with used t-shirts, many commemorating places and events far from the store in space and time. Having so many, they drop the price so that they can sell them quickly.
And here we get to a bit of hipster history. I might be somewhat misled being from Portland, but before hipster fashion became big business, many hipster traits were picked up simply out of expediency. Many of the proto-hipsters that I knew in the proto-hipster days of the 1990s were either students, or worked careers that were less than lucrative. Not having a lot of money, and needing to wear something, stocking up on cheap t-shirts at a second hand store became an obvious money saving option. And many of these t-shirts were, if not ironic, incongruous, since they referred to experiences and places that the wearer had probably not personally encountered.
Much like with the infamous Pabst Blue Ribbon, what was once used for expediency somehow gained a cultural currency. In my mind, this has a lot to do with what was once a subculture with political and social reasons behind it being turned into a subculture more about fashion, which also has to do with its geographic migration. But since I don't want to start that rant, I will continue with our story. Once the idea that a t-shirt was somehow hip, the irony of t-shirts became self-referencing.
And thus is born the second type of ironic t-shirt, the t-shirt that attempts to have an ironic statement about it, rather than just the incongruous t-shirt. And subcultural and media self-recursion being what it is, the subject of the ironic t-shirt is often about the irony of the t-shirt. And this is where people are rightfully scornful of hipsters, especially since < -rant redacted- >. To pay 2 or 3 dollars for a used t-shirt saying that you visited a tourist attraction in New Zealand or were a member of Big Al's Pizza Parlor Bowling Team is an interesting conversation piece, and is at least cheap. To spend 25 dollars to order a t-shirt with the inscription "This is an Ironic T-Shirt" is why people hate hipsters.
But since it is 2010, I will let the reader draw their own conclusions.
Also, right now I am wearing a Perl Monks t-shirt, which while a promotional item, has held up very well for the past seven years.