Irrupt and erupt have existed side-by-side since the former entered the English language in the 1800s (erupt had been a part of the language for over two centuries at that point). Both are descendants of the Latin verb rumpere, which means "to break," but irrupt has affixed to it the prefix ir- (in the sense "into") while erupt begins with the prefix e- (meaning "out"). So "to irrupt" was originally to rush in, and "to erupt" was to burst out. But it's sometimes hard to distinguish the precise direction of a violent rush, and irrupt came to be used as a synonym of erupt in the senses "to become active or violent especially suddenly" and "to break forth."
Example Sentences
Recent Examples on the WebOf a city with clear blue skies, a firm shoreline, and potable water, where large tanks owned by water mafias didn’t roam the streets like predators and sinkholes the size of buildings didn’t irrupt into an ever-rising, salty sea. Usman T. Malik, Wired, 11 Dec. 2020 Much of the excitement comes from the opportunity to see migrants such as snowy owls and winter finches irrupting from the north, as well as southern species expanding their winter ranges northward. James F. Mccarty, cleveland.com, 8 Dec. 2017
Word History
Etymology
Latin irruptus, past participle of irrumpere, from in- + rumpere to break — more at reave