Oh, but English does have a sarcasm marker! It's just not a word, instead it's typically marked by using non-standard pronunciation like creaky voice or lengthening vowels. The problem is of course that this stuff doesn't have an orthography, thus the use of stuff like /s online.

Is it really a sarcasm marker? I've always thought it's equivalent to že in Slavic languages (both probably from the same PIE *g(h)e), and in those languages, it can be used sarcastically, but its main meaning is 'in fact', 'as for X'. For example in Russian: on že glup = 'he is, in fact, a fool' or 'as for him, he's a fool - didn't you know?' Similarly, dictionaries translate the Greek ge as 'in fact, indeed.' If you mentally replace most instances of Ancient Greek ge with the Slavic že in Greek texts, it all starts making a lot of sense (if you're a speaker of a Slavic language).

P.S. There's also a limiting sense in the dictionary, with the example given:

    Greek: ho de ge (+ participle)
    Russian: tot že (kto)
    English: but the one (who)