'Ironic' vs. 'ironical'

The first factor that comes to mind for me here is simply that the latter of these variants--at least in the US--is much more likely to elicit spontaneous merriment. Dictionaries, of course, cannot record this judgment directly (that's my job); and while I've never actually witnessed laughter in connection with the second form, I can certainly imagine it. Webster's New World, 4th ed., does seem to be aware of this state of affairs, giving sole possession of the main entry to 'ironic,' and relegating 'ironical' to the end of the entry. This placement of the latter form, as the preface to WNW 4 explains, implies that it "occurs less often than the main entry or has a special quality, as in being British, dialectal, poetic, or rare."
Merriam-Webster 10, however, does not register this distinction, listing the two as equal variants.

Because the difference in form between 'ironic' and 'ironical' doesn't carry any semantic weight, and neither is widely considered to be an example of substandard/nonstandard/'bad' usage, the choice is theoretically up to a given writer or speaker in determining which form to use. Burchfield suggests that the choice is "governed by the rhythm of the sentence"; it's my conjecture that this is more true in Britain than America, where, it seems to me, 'ironic' clearly prevails in all contexts. I plan to stick with 'ironic,' if for no better reason than not to be laughed off the block. Others can do as they please; but if you come to suffer from the ill effects of using hilarious variant forms, don't say I didn't warn you.