Irrumation is really quite a different thing to fellatio. Both words are from Latin - Fellare and Irrumare. Whilst both activities involve the penis penetrating the mouth, the role of the participants is exactly the opposite.
Fellatio is performed by the owner of the penis lying or standing (or adopting any position for that matter) and being passive whilst his partner stimulates him.
Irrumation is where the owner of the penis is in full control and actively penetrates or fucks his partner's mouth +/- throat. It is what is increasingly more popular in modern pornography and is colloquially known as face/throat fucking or gagging if the recipient's gag reflex is stimulated by the action.