This is it:
Attwood receives the throw and immediately feeds it back to Billy Vunipola. Attwood is then flanked by Haskell and Kruis, and the three of them form a blocking wall that starts moving forward. The Irish don't engage, and when the blocking wall starts moving forward, some of the Irish line backs up a bit until O'Brien dives in. O'Brien then gets caught under the boots, and is penalized for collapsing the maul.
I think what the Irish are thinking is they didn't engage the maul, so no maul is formed and O'Brien can go in on a tackle. But Joubert called it a maul and penalized O'Brien. O'Connell has a talk with Joubert, I think arguing it wasn't a maul, but Joubert doesn't agree (hard to hear that bit; the announcer talks over Joubert, and the conversation isn't in the clip).
Here are my questions:
1.) Is it a penalty to England because the Irish back up after the English start moving forward, or do the Irish just have to hold ground until the ball is fed backwards?
The moment O'Brien binds onto Attwood we have a maul. I think the ball was still with Attwood at this point, however I am not 100% sure about it.
2.) If the Irish were in their rights to back up after Vunipola gets the ball and the blocking line starts moving forward, that's
not a maul, correct? And if that's the case, O'Brien is okay to tackle.
The definition of a maul is 1 opponent and 1 team mate bound onto the ball carrier. As soon as O'Brien tries to tackle the player with the ball we have a maul.
3.) O'Brien getting caught underneath: If it's not a maul, that's O'Brien going in on a tackle, and not completing it. Since he didn't actually make the tackle, can he be penalized for not rolling away? In other words, is he legal down there or not?
See other answers.
In any case, it doesn't look like anyone, including Joubert and the announcers, are on the same page here.