Both provinces reached the Cup quarter-finals last year, and having won and lost against Castres Olympique on the way there, Munster, with former coach Declan Kidney back in situ, have been drawn with the French side again.
The recent Celtic Cup champions were drawn in Pool 1 with Castres, who finished sixth in the French championship, last season's Challenge Cup winners Sale Sharks, and Newport-Gwent Dragons, whom Munster did the double over in the 2004/05 Celtic League.
New Leinster coaches Michael Cheika and David Knox will be facing both Bath and Bourgoin, who were also drawn and beaten both home-and-away by the Irish province last season, in Pool 5. Top-ranked Scottish side Glasgow are the fourth side in Leinster's pool.
Meanwhile, Ulster, whose coach Mark McCall is currently away on Ireland duty in Japan, will challenge Biarritz Olympique, Saracens and Benetton Treviso for a path through to the last eight from Pool 4.
Biarritz, 19-10 quarter-final conquerors of Munster in April, will be favourites, while Saracens are stepping up from last season's Challenge Cup where they were knocked out of the quarter-finals by Brive.
ERC (European Rugby Cup) Chairman Jean-Pierre Lux, who was recently re-elected to his position for a further two years, and ERC Chief Executive Derek McGrath conducted the draw in Cardiff's prestigious opera and concert hall, the Wales Millennium Centre.
2005/06 HEINEKEN CUP POOL STAGES:
POOL 1: Munster, Castres Olympique, Sale Sharks, Newport-Gwent Dragons
POOL 2: Calvisano, Perpignan, Leeds Tykes, Cardiff Blues
POOL 3: Ospreys, Stade Francais, Leicester Tigers, Clermont Auvergne
POOL 4: Biarritz Olympique, Saracens, Ulster, Benetton Treviso
POOL 5: Glasgow, Bourgoin, Bath Rugby, Leinster
POOL 6: London Wasps, Toulouse, Llanelli Scarlets, Edinburgh
Pool stages: Round 1 - Oct 21/22/23, 2005 Round 2 - Oct 28/29/30, 2005 Round 3 - Dec 9/10/11, 2005 Round 4 - Dec 16/17/18, 2005 Round 5 - Jan 13/14/15, 2006 Round 6 - Jan 20/21/22, 2006
Knock-out stages: Quarter-finals - Apr 1/2, 2006 Semi-finals - Apr 22/23, 2006 Final (Millennium Stadium, Cardiff) - May 20/21, 2006