Sean O'Brien confirmed that in an interview in the Times today:
Seán O’Brien, along with most others involved, believed the try was legitimate until the slow motion replays showed that Sexton was absolutely correct and Le Roux had indeed dropped the ball just before impact.
“I was cursing at the time, saying for f**k’s sake,” said the Leinster and Ireland flanker O’Brien. “Then Johnny said to Nigel to check it. He wasn’t going to check it . . . It came up on the big screen then and I said ‘he’s after dropping it, yeah’."