Irony means using a word or phrase to mean the opposite of it's literal meaning. Like, "Well, THAT was a very nice thing to say" when someone says something rude. Or it can mean an event the opposite of what was expected or intended, like "How ironic that Jeff's efforts to get a promotion resulted in him being fired." So I don't see any irony here. I'm not sure what the opposite of "vessel" would be, but I don't see any sense in which the word is being used to mean the opposite of what you'd expect. This is a metaphor, not irony.
Frankly I don't understand the point of the poster's comment. Clearly here "vessel" is NOT being used literally -- a vessel is not just a vessel. Paul is not talking about water jugs, he is talking about human bodies.
In context, well, I guess you could make the argument about Paul using "vessel" to mean "wife". I find that unlikely here, but perhaps you could make a case for it. Barring that, whether he is using "vessel" to refer to the whole body or to sex organs in particular is a moot point. Either way, he's talking about sexual behavior, so it doesn't change the meaning. Ultimately, he clearly and plainly IS talking about sex organs.
(It seems like the writer is over-studying the metaphor just so he can build up to a mildy crude comment for, what, mild shock value? It seems a little silly to me. I can just see the author giggling to himself as he says, "Hee hee, I wrote the word 'pecker' on a theology site". Is he 12 years old? Note to people who like to shock: Inserting sexual comments into a discussion about math or gardening may shock people and/or give you a thrill. Inserting sexual comments into a discussion that is already about sex does neither. Oh well, I'm getting off topic.)