The trophy, which is currently retained by Old Wesley, will be contested by hosts Blackrock College and the eight teams that qualified into the play-offs of the Frazer McMullen Cup competition earlier this year. The tournament will convene on an annual basis.
Speaking at the launch, Fergus Slattery said: "It has been a long time coming, but it is fantastic to see the return of the Castle trophy.
"To secure the competitive future of Irish rugby, it is imperative that we give young players the opportunity to compete at a high level and showcase their skills. I firmly believe that this tournament will set the standard."
Tom Browne, Chief Financial Officer of sponsors Friends First, spoke proudly of their decision to support the event,
"Friends First is delighted to sponsor the Castle trophy tournament for the next three years," he announced.
"Together with Blackrock College RFC, we expect it to become the pre-eminent Under-21 tournament in Ireland, setting new standards in rugby excellence. We look forward to seeing some of these players compete at the top level of Irish rugby in coming years."
As well as the Castle trophy, there will be a host of other events taking place at Stradbrook over the course of the weekend.
On Sunday, September 23, there will be an exhibition of touch rugby, a women's All-Ireland League match and the Eric O'Connor Cup - an Under-11/Under-12 rugby blitz in memory of Eric O'Connor, an enthusiastic member of the Blackrock Under-11 Minis team, who sadly passed away earlier this year.
The teams competing in the Friends First Castle trophy are UCD, Lansdowne, Old Belvedere, St. Mary's College, Old Crescent, Garryowen, UL Bohemians, Old Wesley and Blackrock College RFC.
For further information on the Friends First Castle trophy, please visit www.blackrockcollegerfc.ie.