Kavanagh is called onto the replacement bench in place of Oisin Hennessy(Dungannon) but other than that McDermott goes with the players who saw off the Tongan challenge on Friday night last.
The Irish face a far more difficult task next Tuesday against an Argentine side who are seeded 3 in the competition and who defeated Ireland this time last year. They include three playrs from that squad and they opened their campaign with a seven try 41-10 victory over Italy.
Meanwhile Shaun Payne has been called up to the senior squad as replacement for Gordon D'Arcy who was forced to retire with a dmaaged groin in the first Test last Saturday.
Payne was already in South Africa on vacation and was in fact a spectator in Bloemfontein when D'Arcy suffered his injury. Payne joins the Irish squad today in Cape Town while D'Arcy returns home.
Ireland Under 21:
15. Andrew Finn (Dolphin) 14. Richard Lane (UCC) 13. John Hearty (Blackrock Coll) 12. Glen Telford (Dungannon) 11. Tommy Bowe (Queens Uni) 10. Gareth Steenson (Queens Uni) 9. Tomas O'Leary (Cork Constitution) 1. John Wickham (Clontarf) 2. Denis Fogarty (Cork Constitution) 3. Declan Fitzpatrick (Belfast Harlequins) 4. David Gannon (Blackrock Coll) capt 5. Shane O'Connor (Cork Constitution) 6. Brendan O'Connor (Cork Constitution) 7. David O'Brien (Old Belvedere) 8. Jamie Heaslip (Dublin Uni)
16. Conor Geoghegan (UCD) 17. Keith Doyle (UCD) 18. Neil McComb (Dundee HSFP) 19. Anthony Kavanagh (Garryowen) 20. Robbie Shaw (Wasps) 21. Michael Glancy (Loughborough Uni) 22. Breiffne O'Donnell (UL Bohemians).