Or did they all fall over the original white ball carrier who was also on the ground? Hard to tell in slo-mo let alone at full speed.
This part is not in line with 2014 instruction.
When O'Brien made contact he formed a maul, as defined in Law 17. Other Irish players joined in, O'Brien was on the ground and the advancing maul stumbled over him. He was penalised for causing the maul to collapse
According to the 2014 instruction, O'Brien was entitled to tackle the ball carrier and therefore his first contact could not be seen as forming the maul.
1) If the defenders in the line out choose to not engage the line out drive by leaving the line
out as a group, then PK to attacking team.
2) If the defenders in the line out choose to not engage the line out drive by simply opening up
a gap and creating space, and not leaving the line out, the following process would be
followed:
a) Attackers would need to keep the ball with the front player, if they were to drive downfield
(therefore play on, general play - defenders could either engage to form a maul,
or tackle the ball carrier only)
b) If they had immediately passed it back to the player at the rear of the group, the
referee would tell them to "Use it" which they must do immediately...
c) If they drove forward with the ball at the back (and did not release the ball), the
referee would award a scrum for "accidental offside" rather than PK for obstruction.