The wiki definition of irrumatio distinguishes it from fellatio on the basis of the penis moving instead of the head moving. A quick look through the irrumatio tag reveals very very few images where it is clearly the penis or hips moving. Most of the time it's either impossible to tell, there's no motion at all, or it's actually obviously the head moving.
The actual criterion people have been using to tag irrumatio is pretty clearly the presence of hands on the fellator's dick-sucker's head, indicating that the blowjob is being forced or controlled by the person with the dick. Given that, should irrumatio be redefined (or renamed) as something more like forced_fellatio?