Having been beaten 14-3 at Thomond Park they extracted ample revenge with a 33-10 victory. It meant that for only the second time in 17 years Munster didn't feature in the knock-out stages and suffered their worst European defeat since 1996.
O'Mahony recently described that defeat as "stomach-churning" and doesn't want a repeat. Munster have certainly improved after two years out of the knock-out stages, but so have Saracens.
Mark McCall's men go into this weekend's semi-final, a record fifth in a row, needing one more win to make it 17 games without defeat in the tournament. Their aim over the next month is simple - to complete the double-double by retaining their Champions Cup and Premiership titles.
"We always pride ourselves on how we do in Europe and we let ourselves down over the past couple of years. And that defeat to Saracens was particularly tough to take," admitted O'Mahony.
"But we have a different group now and we are a different animal. We've learned a lot of lessons and we now go about our business in a different way.
"Under the new coaching staff we have put an emphasis on enjoying our rugby. We know we are going to have our work cut out this weekend, but we want to embrace the challenge and enjoy it because these are the things you are going to look back on.
"Whether you win, lose or draw, you look back on these weekends because they are obviously the biggest weekends of your life. You want to look back on them with fond memories."
There is no doubt a victory over the best team in Europe and, according to Munster director of rugby Rassie Erasmus, one of the best he has seen in either hemisphere, would be right up there in the greatest achievements in Munster's European history. After four successive defeats in European Cup semi-finals, and a Challenge Cup semi-final reverse at Thomond Park, the time feels right for a return to the final stage after a gap of nine years.
Saracens know they will be up against it in Dublin - Munster have distributed 37,000 tickets and the Aviva will be a sea of red. But that's just how the Saracens like it, backs to the wall, testing and hard.
They have received a double boost in the past week with the return to action of England lock George Kruis and the announcement of six of their players in the Lions squad to head to New Zealand this summer - Owen Farrell,
Mako and Billy Vunipola, Jamie George, Maro Itoje and Kruis.
The battle between new Lions No 8s Billy Vunipola and CJ Stander, if fit, will be crucial to the outcome of the game and O'Mahony and Donnacha Ryan's line-out work will need to be at its very best to mach Kruis and Itoje. Tyler Bleyendaal will need to match or better Farrell in the goalkicking department and Simon Zebo will have to not only keep a close eye on try machine Chris Ashton, but try to add to his 19 tournament tries into the bargain.
Both teams got out of gaol last weekend in their respective domestic leagues, Munster beating Ulster Rugby 22-20 in Limerick, while Sarries hauled themselves past Northampton Saints 27-25. That performance in Milton Keynes kept them on course for a hoe semi-final in the Premiership, but angered their direcgor of rugby.
"It was a bit of a lesson to us about the big things that need to be right. Some of those big things that were wrong won't be good enough against Munster and we need to work hard on them during the week," said McCall.
That just about sums up Saracens - always striving to get better. Just how much Munster have improved under Erasmus this season will be revealed over 80 minutes (or more possibly) this weekend.
- This will be Munster's 12th European Cup semi-final - a record for the competition - while Saracens have reached the semi-finals for a sixth time and for the fifth season in a row.
- Munster will be playing their 158th tournament game - the most of any club. The two-time champions are currently level with Toulouse, who they beat in the quarter-finals, on 157 matches.
- This will be the 8th meeting between the clubs; Munster have won five of the previous seven clashes (L2), although those two defeats against Saracens have come in their last three meetings.
- These sides have met once before in the semi-final with Munster progressing to the final in 2008 after an 18-16 victory at the Ricoh Arena before going on to lift trophy in Cardiff.
- Saracens are unbeaten in their last 16 Champions Cup games (W15, D1), and only Leinster have gone on a longer unbeaten streak in the competition (W16, D1, 2010-2012).
- Munster have won their last four Champions Cup games and have not gone on a longer run since winning six-in-a-row in 2013/14 when they last reached the semi-final.
- Chris Ashton needs just one try to become the outright top try scorer in European Cup history. He is currently level with Vincent Clerc on 36 tries.
- Munster (5) and Saracens (8) have conceded fewer tries than any other side this season, with both teams yet to concede a first-half try.
- CJ Stander has made 114 carries this season, and no other player has made 100+ carries so far in the competition.
- Owen Farrell (97) and Tyler Bleyendaal (94) are the top point scorers in the Champions Cup this season, and no other player has scored more than 80 points to date. Farrell was last season's top scorer with 127 points.