McNamee is set to be included in Irish coach Mark McDermott's IRB Under-21 World Cup squad, which is announced tomorrow, for the tournament in Argentina next month.
The 20-year-old Northampton-born starlet said: "This really is a dream come true for me. I have supported Saints all my life and watched the club play from the terraces. It is a great opportunity and a great honour to officially become a part of the first team squad."
Northampton Academy coach Rob Hunter told the club's website: "Brett has worked incredibly hard to earn this chance. He is extremely professional and has a huge desire to play for Northampton. He plays with heart, is tough and committed. We wish him the best of luck in his professional career both with Saints and Ireland."