The best description that I can find is for an own label version which says "Carbonated mixed fruit flavour soft drink with sugar and sweeteners" and I belive it contains caffine, quinine and some thing called Ammonium Ferric Citrate what ever that is. I haven't tasted it for over 50 years and hated as a child. I know that they had to reduce the amount of sugar that it contained a few years ago - so die hard fans think it isn't the same.
Own label ingredient list is -
Carbonated Water, Sugar, Citric Acid, Flavourings (contains Caffeine), Fruit and Vegetable Concentrates [Safflower, Lemon, Black Carrot, Blackcurrant], Acidity Regulator (Sodium Citrates), Preservative (Potassium Sorbate), Sweeteners (Sucralose, Acesulfame K), Ferric Ammonium Citrate.
Actual Irn Bru says -
Carbonated Water, Sugar, Acid (Citric Acid), Flavourings (including Caffeine, Ammonium Ferric Citrate & Quinine), Sweeteners (Aspartame, Acesulfame K), Preservative (E211), Colours (Sunset Yellow FCF, Ponceau 4R)
Query on the internet says "It's orange, fizzy and tastes like liquid bubble gum. And while whisky is Scotland's national drink to the world, to many locals, Irn Bru is a close second" also it contains 32 flavours. As well as someone said tastes similar to an orange cream soda, Circus Peanuts in the US or contains banana flavour. A German said it was like a German tutti fruitti drink but no more description that that.
I hope that gives you some ideas.