I'm trying to come up with a definition of morph, morpheme, and allomorph for you, but everything I think of is just more confusing.
Basically, Jonathon is saying they're all forms of the same prefix that have changed to become more easily pronouncable. Like, "in-mature" would be much more difficult to say than "immature," and "in-resposible" would be harder to say than "irresponsible." (You don't know how hard it was for me not to use irregardless as my ir- example.)