Why does Irn-Bru taste better slightly chilled?

A few things happening at once:

1. Carbonation holds better cold. CO₂ is more soluble in cold liquid, so the fizz is tighter and more consistent. At room temperature it goes flat faster and the bubbles feel coarser.
2. Sweetness is suppressed. Irn-Bru is extremely sweet (the original has more sugar per can than Coke). Cold temperatures blunt sweet taste receptors, so the flavour feels more balanced rather than cloying.
3. The artificial notes are less prominent. The distinctive Irn-Bru flavour — that hard-to-place synthetic bubblegummy thing — contains volatile aromatic compounds that are more aggressive at room temperature. Chilling reduces their volatility so they hit the nose less and blend more smoothly.
4. Contrast and refreshment. Cold drinks register as more refreshing partly because of the physical temperature contrast and partly because lower temps slow saliva production, making the acidity feel cleaner rather than coating the mouth.

"Slightly chilled" (not ice-cold) is the sweet spot because full fridge-cold starts to suppress all flavour, including the good stuff.