No doubt true for “inflammable”/“flammable”, but I would be shocked to see even a single example of someone sincerely employing “irregardless” to mean the opposite of “regardless” (e.g., “Though the project was otherwise a smashing success, irregardless to the excessive final cost, it would have to be described as a mixed victory at best”), entirely unaware that this is at odds with the majority of other occurrences of the word.
Putting aside “pressurising”, if your implication is that there is something wrong with that use of “careening”, I’m not sure exactly what the objection would be. At any rate, the American Heritage Dictionary gives the usage note