Irradiation is a short and "improper" term for multiple internal reflections. It is caused by turbidity which is the opaqueness of the emulsion making it cloudy instead of clear. This is reduced by acutance dyes put into the emulsion. They are the things that wash out when you process commercial emulsions. They slow the emulsion down.

AH layers, for all practical purposes, do not slow down an emulsion.

Often, what appears to be "irradiation" or halation is caused by UV which varies with season, time of day and lens. Modern coatings contain UV overcoats to minimize or "nrmalize" this problem.

The orance or black septom decreases the effect of back reflections, but as Denise points out, this is not complete.

Collodion contains ether and is not recommended for the average household darkroom.