"The future of Irish rugby depends on events like this that give young players the opportunity to compete at a high level and showcase their skills. The success of last year's event set the standard, but I am confident that we can emulate it this year."
After a 46-year hiatus, the tournament returned last year to mark Blackrock's 50th anniversary at their home ground. It has now become an annual event in the rugby calendar.
The trophy will be contested by last year's winners Old Belvedere, Lansdowne, UCD, Trinity College, UCC, Old Wesley, Garryowen and hosts Blackrock.
"Friends First is delighted to continue our sponsorship of the Castle Trophy tournament," said O'Neill.
"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."
There will be a number of other events taking place at Stradbrook over the course of the weekend including the Eric O'Connor Cup - an Under-11/12 rugby blitz in memory of Eric O'Connor, an enthusiastic member of the Blackrock Under-11s Minis team who sadly passed away last year.
For further information on the Friends First Castle Trophy, please visit www.blackrockcollegerfc.ie.