The sides were level (6-6) at the break before the home side delivered a much improved second half display to run in three tries, from Fergus Gately, Emmet McLoughlin and replacement Donnacha Ryan.
Out Half Barry Keeshan converted each of those scores having earlier landed a drop goal and a penalty in the opening half. Munster travel to Belfast next weekend for the return in Ravenhill with the destination of the trophy firmly in their own hands.
Final Score: Munster 27 Ulster 6
Munster U21 Team:
Cathal Garvey (UL Bohs ); Andrew Finn (Dolphin), Brian Hastings (Trinity), Breffne O Donnell (UL Bohs), Paul Hurley ( UCC); Barry Keeshen (UCC), Thomas O Leary (Cork Con), Fergus Gately (UCC), Robert Quinn UCC), Emmet McLoughlin (Shannon); Ryan Hartigan (UL Bohs), Shane O Connor (Cork Con); Mark Melbourne (Garryowen), Anthony Kavanagh (Garryowen), Brendan O Connor (Cork Con). capt
Replacements: Killian Barry (UCC) for O'Leary; Donnacha Ryan (UCC) for Hartigan; Damien Varley (Garryowen) for Quinn. Not Used: Ger Collins (Bruff), Rory Brosnan (Garryowen), Eddie Molloy (Dublin University), Brian Carroll (Bruff), Michael Hastings (UCD)
Richard McCarter (QUB); John Hill (Malone), Andrew Trimble (Ballymena), Neil Patterson (Dungannon), Paul McKenzie (Loughborough University); Gareth Steenson (QUB), Michael Kirkwood (B. Harlequins); Niall Conlon (Dublin University)capt Philip Currie (Portadown), Leslie George (Coleraine); Lewis Stevenson (B. Harlequins), David Houston (Ballymena); Glenn Moore (QUB), Oisin Hennessey (), Chris Henry (Malone).
Referee - C Stanley (Leinster).