"Ir- is the assimilated form in Latin of the prefix in- before initial letter r, used in the same way in English (and much more frequent than in-) in words such as irrational," as well as irrecoverable, irreligion and irrecognizable. OED

Precisely.

"Elided"? No, that means leaving out a vowel; in this case it's a consonent that's left out. I suppose that dictionary definition above indicates that "assimilated" is the correct technical term. I think "eroded" is a fair enough word to refer to the process by which we knock the rough edges off words that are tricky to pronounce.