According to Wikipedia, the undisputed and infallible source of all knowledge:
Irn-Bru is a Scottish carbonated soft drink, for long advertised as "Scotland's other national drink" (next to Scotch whisky). It is produced in Westfield, Cumbernauld, North Lanarkshire by A.G. Barr of Glasgow, since moving out of their original Parkhead factory in the mid-1990s, and at a second manufacturing site in Mansfield, England. In addition to being sold throughout the United Kingdom, Barr's Irn-Bru is available throughout the world and can usually be purchased where there is a significant community of people from Scotland. Innovative and sometimes controversial marketing campaigns have kept it as the number one selling soft drink in Scotland, where it competes directly with global brands such as Coca-Cola and Pepsi.I discovered IRN-BRU about two weeks ago, and I've been drinking it every couple of days since then, trying to decide what I think of it. It's not bad, I can say that much. I'll drink Pepsi on occasion, I almost never drink Coke, but IRN-BRU doesn't really have a "cola" taste to it, which helps. It's pretty sweet, and even the sugar free version (which I accidentally drank a few days ago) is still drinkable.
Also, if you're ordering it at the pub, don't call it "urn-brew", or they'll probably make fun of you.