The two men worked together as head coach and captain with Ireland
Brian O'Driscoll's attacking skills are widely known and admired.
In 141 Test matches for Ireland and the Lions he scored 47 tries. He is 16 tries ahead of Tommy Bowe in the list of top try-scorers for Ireland.
Despite O'Driscoll's flair with the ball, it was his time without the ball that most impressed former Ireland head coach Eddie O'Sullivan. The former Leinster centre won 38 of his caps for Ireland under O'Sullivan.
Speaking to Jarlath Regan's An Irishman Abroad podcast, the former Ireland head coach said that O'Driscoll was the one player that consistently took his breath away.
"We've seen the fantastic things he's done, like when he scored tries," O'Sullivan said in the hour-long conversation. "The part of his game that I enjoyed watching the most was his defence.
"Everybody watched him with the ball, that was lovely. Defensively, he's the greatest defender I've ever seen in rugby. He was the smartest defender, but by God was he tough. He'd take anybody down. He always knew which guy was going to get the ball so he could tackle them.
O'Driscoll's heroics in Paris in 2006 impressed O'Sullivan. Mandatory Credit ©INPHO/Andrew Paton
"People always remember Brian for tries he scored and how important they were, but also for how tough he was, physically and mentally. The bigger the occasion, the harder he played.
"I've seen him get injured in games and he wouldn't come off, and he should have come off. He just wasn't going to come off because we needed him."
O'Sullivan pinpointed one game, where he felt O'Driscoll's bravery truly shone, despite Ireland losing the match.
"I remember in Paris in 2006, we had a catastrophic start to the game. He got a dreadfully bad dead leg. The muscle was bleeding from the thud he got from a knee. He played through for an hour with that. He could have come off, and he should have come off, but he knew we were in trouble."
Ireland lost that game 43-21, but O'Sullivan revealed that O'Driscoll and Paul O'Connell both were injured and refused to come off, despite trailing in the match.
You can listen to the full conversation here.