Why is there an h in 'ihr'?
I don't understand why the letter h is in the word 'ihr'. As far as I know 'Ir', without an h, would be pronounced the same. Is this just some piece of outdated spelling that was never changed, or do I not understand something?
You are absolutly right. Sometimes it is to differntiate words which are spoken the same way, like malen and mahlen but I don't know any word ir. I think it has historical reasons as there existed several ways to indicate long vovels before sticking to the multiple consonant-rule, all of them left remanants. So the Dehnungs-h before l,m,n,r are still here after the rule that several consanants make a voval short was established and consononats doubled. Somtimes you will find the Nieder-German system that doubels the vovel as in Geest what would be in modern writing Gehst and is spoken as the verb and even earlier one added an i or e to indicate lenght like in the German city of Soest (no ö). That method conflicted with ä, ö, ü alternative writing, so only the ee and ie are left.
Even today many German dialects have more than two vovel lengths, so possibly those before l, m, n, and r were felt longer than the normal long ones in those days.
What I can say is, that those long vovels indicated with an h are never spoken short, how it can happen with others (was, das).