outbreak vs irruption

outbreak and irruption both are nouns.

Word	Noun	Adjective	Verb	Adverb
outbreak	Yes	No	No	No
irruption	Yes	No	No	No

As nouns, outbreak and irruption are synonyms defined as:

outbreak and irruption: a sudden violent spontaneous occurrence (usually of some undesirable condition)

Other synonyms of outbreak include eruption.

outbreak (noun)	irruption (noun)
a sudden violent spontaneous occurrence (usually of some undesirable condition)	a sudden violent entrance; a bursting in
	a sudden violent spontaneous occurrence (usually of some undesirable condition)
	a sudden sharp increase in the relative numbers of a population