Learn English – the meaning of “hypos” here in this passage from Moby Dick

Definitions of irony (and there are quite a few) emphasize the notion of incongruity—between something expected and something that comes to pass, for example, or between something said or done in a dramatic situation and how the action is understood by the audience. Here is the entry for irony in Merriam-Webster's Eleventh Collegiate Dictionary (2003):

irony n (1502) 1 : a pretense of ignorance and of willingness to learn from another assumed in order to make the other's false conceptions conspicuous by adroit questioning — called also Socratic irony 2 a : the use of words to express something other than and especially the opposite of the literal meaning b : a usually humorous or sardonic literary style or form characterized by irony c : an ironic expression or utterance 3 a (1) : incongruity between the actual result of a sequence of events and the normal or expected result (2) : an event or result marked by such incongruity b : incongruity between a situation developed in a drama and the accompanying words or actions that is understood by the audience but not by the characters in the play — called also dramatic irony, tragic irony

It seems fairly clear than nothing in this entry makes ironically a good fit for the sentence quoted in the OP's question. So what was the intended word? I suspect that the author might have been looking for coincidentally. Again from the Eleventh Collegiate:

coincidentally adv (1837) 1 : in a coincidental manner : by coincidence {lonely singles who meet coincidentally and click — People} 2 : it is or seems coincidental that {coincidentally, the dog died exactly one year after his owner did}

In the example, there is a kind of coincidence in the fact that launch was delayed by an unsafe condition (the sailboat's presence in the rocket's danger zone) and the fact that the explosion of the rocket was precisely the kind of potentially disastrous occurrence that led the government to delay launches when the boats are in the area. But this is not a case of incongruity between launch policy and launch result; instead it's a clear case of congruity between the two. It follows that ironically is not a suitable word to describe the relationship between the launch delay and the launch explosion.

"May be not," taking out a stick and whittling a toothpick, "but I rayther guess you'll be done BROWN if that ere harpooneer hears you a slanderin' his head."

"I'll break it for him," said I, now flying into a passion again at this unaccountable farrago of the landlord's.

Moby-Dick (Chap. 3: The Spouter-Inn)

I'll break his head.

It means that he will hit or punch the man and break his skull. You have to read further to discover what the landlord is talking about.