Comically antiquated variation on 'ironic' presently used most often to emphasise the unseriousness of a point. 'Ironical', like many nouns accepting the -ical adjectival suffix (e.g., poetical, hermetical, etc.) fell into disuse in favour of the shorter -ic form (e.g., poetic, hermetic, etc.) in Early Modern English. The anachronistic 'ironical' is therefore most commonly used in current speech to suggest the absurdity of an expression.
Speaker 1: That...that doesn't make much sense.
Speaker 2: Yes, one might even suspect, for example, that I was being ironical!