One thing is the inconsistency in regards to pluralization. For example, one says "children" instead of "childs" but "rabbits" instead of "rabbitren. read more
To be honest I think Italian ( even if being a latin language it has some irregular points) is not irrational at all. It's a language made of fixed rules and most of the times is enough to apply the rules in order to build sentences. As Caterina said in the country there are so many dialects, but this is another fact. read more
There aren't many "irrational" things -- the English language has developed into what it is. The spelling rules are arguably irrational, yet that is an expected result of a language that incorporates so many things from other languages. Pronunciation is arguably irrational too -- again, the expected result of multilingual incorporation. read more