Encarta:

irrupt | irrupted [past tense] | irrupting [present participle] | irrupts [3rd-person singular present]

Summary:

to enter suddenly or violently | to increase suddenly and rapidly, e.g. in number  (13 of 58 words, 2 senses, 1 etymology, pronunciation)

Link:

http://encarta.msn.com/dictionary 1861622514/definition.html  [cite]

Wiktionary:

irrupt | irrupted [past tense] | irrupting [present participle] | irrupts [3rd-person singular present] | irruptive

Summary:

[transitive verb] To break into. | [intransitive verb] To enter forcibly or uninvited. | [intransitive verb] To rapidly increase or intensify. | Misspelling of erupt.  (22 of 24 words, 4 senses, 1 etymology)

Link:

http://en.wiktionary.org/wiki/irrupt  [cite]