The Collaborative International Dictionary of English v.0.48:

Irruption \Ir*rup"tion\, n. [L. irruptio: cf. F. irruption. See
   Irrupted.]
   [1913 Webster]
   1. A bursting in; a sudden, violent rushing into a place; as,
      irruptions of the sea.
      [1913 Webster]

            Lest evil tidings, with too rude irruption
            Hitting thy aged ear, should pierce too deep.
                                                  --Milton.
      [1913 Webster]

   2. A sudden and violent inroad, or entrance of invaders; as,
      the irruptions of the Goths into Italy. --Addison.

   Syn: Invasion; incursion; inroad. See Invasion.
        [1913 Webster]

WordNet (r) 3.0 (2006):

irruption
    n 1: a sudden violent entrance; a bursting in; "the recent
         irruption of bad manners"
    2: a sudden sharp increase in the relative numbers of a
       population
    3: a sudden violent spontaneous occurrence (usually of some
       undesirable condition); "the outbreak of hostilities" [syn:
       outbreak, eruption, irruption]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "irruption":
   air attack, air raid, air strike, blaze, boarding, burst,
   convulsion, cyclone, encroachment, entrance, entrenchment,
   eruption, escalade, explosion, fire raid, fit, flare-up, foray,
   gale, gust, hurricane, impingement, imposition, incursion,
   infiltration, influx, infringement, injection, inroad, insinuation,
   interference, interjection, interloping, interposition,
   interposure, interruption, intervention, intrusion, invasion,
   obtrusion, outbreak, outburst, paroxysm, raid, razzia,
   saturation raid, scaling, seizure, shuttle raid, spasm, storm,
   tempest, tornado, trespass, trespassing, unlawful entry, upheaval,
   whirlwind