eruption / irruption are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
eruption: (noun) symptom consisting of a breaking out and becoming visible. (noun) (of volcanos) pouring out fumes or lava (or a deposit so formed). (noun) the emergence of a tooth as it breaks through the gum.
irruption: (noun) a sudden violent entrance; a bursting in. (noun) a sudden sharp increase in the relative numbers of a population.
eruption: ih·r·AH·p·sh·ah·n
irruption: NA
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)