No, "Irn" is not a spelling mistake..
Many of my friends will know that, sometimes, my cupboards reveal some pretty ancient food-stuffs and with the UK's obsession with 'Sell-by' dates (led by the other obsession of suing anyone beacuse we're too stupid to think for ourselves!) these items can sometimes be a little 'old..'
My own personal favorite is my tin of Gales Honey with a Sell-by of 1994 which I deliberately brought from London when I moved) but the other day I found a can of Irn-Bru (Made in Scotland from Gerrrrr-de-rrrrs - brilliant advert!) with a sell-by date of June 2005 which means it was probably bought in 2003 or 2004 while I was working in Glasgow and regularly drinking the stuff but obviously not drinking enough of it or drinking it fast enough!
(btw, the reference to 'Any Old Irn' is to "Any ol' iron', a song from ages ago - I was trying to find out more but gave up!)