The irrumator may be the dominant party or the party getting the most sensation but in terms of activity is (usually) the least active- I would say passively enjoying good fortune while the other party (irrumatee?) is doing all the hard labor

It's part of the definition of irrumatio (face-fucking someone, active) as opposed to fellatio (performing oral sex on a man, passive). In other words, one performs irrumatio, but receives fellatio. Having your mouth involved in either act was considered degrading in Roman times, but submitting to the former more so because of its ultimate passivity.