The words eruption, irruption sound the same but have different meanings and spellings. Why do eruption, irruption sound the same even though they are completely different words?
The answer is simple: eruption, irruption are homophones of the English language.
The act or process of erupting.
An instance of erupting.
A sudden, often violent outburst.
An appearance of a rash or blemish on the skin.
The action of irrupting or breaking into; a violent entry or invasion; an inbreaking; an intrusion.
An abrupt increase of numbers of a particular animal.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").