Matt Williams - who was denied 22 players to choose from - was obliged to field the most inexperienced team of his reign at Longford on Saturday against an English Premiership side who were roughly at half-strength.
Williams was upbeat about the performance which, into a second-half wind especially, deserved a win. "It was disappointing to lose it in the last seconds but to come back into that strong wind in the second-half showed real courage, fitness skill and desire."
Leinster made more of the wrong decisions and handling errors, but they also made more of the play, and with more penetration would have made this tell.
Both of their scores came from close-in moves off the base of the scrum. With Bristol down to 14 men due to the sinbinning of their lock Andrew Sheridan, Ben Willis sniped on the blind side and popped the ball in the tackle for Des Dillon to plunge over.
In the second-half Dillon picked up from the back of a scrum and took out two players to offload in the tackle, the supporting Shane Jennings scoring adroitly.
The first cancelled out a soft try by Martin Shaw, and the second an opportunist try at the base of a Leinster scrum by Peter Richards, but Bristol had the final say when Shane Drahm dummied over.
Scoring sequence: 5 mins Dunne 3-0; 10 Shaw try, Drahm con 3-17; 24 Dillon try, Dunne con 10-7; (half-time 10-7); 44 Richards try, Drahm con 10-14; 71 Jennings try, Leek con 17-14; 80 Drahm try and con 17-21.
LEINSTER: P McKenna; G Browne, K Lewis, C Warner, S Keogh; A Dunne, B Willis; J Lyne, G Hickie, N Treston, B Gissing, A Kearney, A McCullan, D Dillon, L Toland (capt). Replacements: D Quinlan for Dunne (half-time), S Jennings for Toland (35-38, 65 mins), E Bohan for Lyne (65 mins), M Leek for Warner (65 mins), F Baynes for McKenna (74 mins), D Sheahan for Kearney (74 mins), D Blaney for Hickie (83 mins), S Whelan for Willis (83 mins).
BRISTOL SHOGUNS: L Best; J Williams, D Rees, M Shaw, P Christophers; S Drahm, P Richards; P Johnstone, S Nelson, E Bergamasschi, A Sheridan, A Sheridan, S Morgan, B Sturnham, R Beattie (capt), C Short. Replacements: B Daniel for Rees (half-time), M Salter for Beattie (60 mins), C Rowland for Nelson (70 mins), R Oakley for Short (70 mins), D Hayward for Shaw (83 mins).
Referee: O Trevor (IRFU).