Bursting onto the professional scene in 2006, the powerful number 8 Diack, formerly of South African side the Stormers, has been cited as a player of significant potential.
Standing at 6ft 6in, Diack will bring a substantial physical presence to Ulster's back rower. A solid player with an intelligent reading of the game and outstanding all round skills, he has a big future ahead with Ulster.
Speaking about the young back rower, Ulster coach Matt Williams commented: "Robbie has made a really big impact for the Stormers and is definitely coming into the best form of his career. He will add some real strength to the back row and is an explosive and smart player."
When asked about his move to Ireland and Ulster Rugby, Diack commented: "I'm extremely excited about the move to Ulster. My experience playing for the Stormers and Western Province over the past three years has been fantastic and I feel 100% positive that my move to Ulster will be just as good and it will help mould me into a better rugby player.
"What I'm looking forward to most is the different style of rugby played in Ireland and on top of that, meeting and learning from new coaches and players.
"Moving to a new country for me is also an exhilarating aspect, as I will get to experience a totally different culture and learn from it. I suppose in a way moving to Northern Ireland and not knowing any one will also force me to get to know different people, and make new friends, which is always great to have," he added.
Cillian Willis is also shaping up to be a star of the future for both province and country. At 23 years of age, Willis has won numerous caps for Ireland Under-19s and Under-21s, while impressive form since his Leinster senior squad debut in 2006 resulted in a call-up for the Ireland 'A' squad.
The Dublin-born scrum half has moved out of cousin Brian O'Driscoll's shadow at Leinster. He won a 'Player of the Month' award after a string of impressive performances.
Willis' swift passing, sharp tackles and great maturity on the pitch will make a significant impact on the Ulster squad in the new season.
Talking about his move north, Willis remarked: "I'm really delighted to be joining Ulster Rugby, it is an exciting time for the province with a young squad and new coach and I'm looking forward to being part of it next season.
'Ulster has some of the most loyal supporters and they have made Ravenhill a very intimidating place to play. It will be good to experience that atmosphere at the ground and this time be on the right side of it.
"I've met with Matt Williams a few times recently and I'm thrilled about working under such a well respected coach, the lads that I have played alongside at Leinster all speak very highly of him. There's going to be competition for places and Isaac (Boss) and Paul (Marshall) are two very bright players.
"I know Paul quite well, we played together at underage levels so that will be fun and I've talked to Isaac a bit recently. I'm really looking to learn from him and Paul over the next year."
Steve Williams, assistant coach of Ulster Rugby, said of Willis: "Cillian is a young player who has shown great promise, both early in his career with UCD and in the last few years with Leinster and the coaching team at Ulster are very excited to have him on board for the new season.
"We will look to expand Cillian's skills and push him to the next level in his game play - he will also add some competition for position into the team, which will help existing players tighten up their game."