There has been a belief, at times wildly popular, the using a half full can of beer (or soda, or other beverage) as a base when barbqecuing or roasting chicken helps keep it moist, and imparts flavor.
This is pretty much debunked. As Meathead says in the linked article:
No way the beer adds moisture. The method is supposed to add moisture to the meat. But the can is inserted half way up the cavity and blocks the meat from contact with moisture. It acts like a condom. so any vapor that escapes the top of the can, and there isn't much, will only come in contact with the upper half of the cavity. The surface area of the exterior of the bird is vastly greater than the surface area of the cavity, and after blocking off half the cavity with the can, there is very little surface area for flavor to penetrate.
He also explains in detail why beer cannot add much flavor, if any--there are very few flavorful ingredients in beer as a proportion of the whole which is mostly water. While I am not familiar with Irn Bru, I imagine that most or all of the logic still applies.