After the victory over Northampton the 'Bull' received his 100th cap and a commemorative piece of crystal from ERC Chief Executive Derek McGrath and Heineken's Pat Maher, while match winner Ronan O'Gara, who had just made his 98th tournament appearance, looked on.
Click here to view the Bruff clubman's ERC profile, with his list of matches stretching back to his debut in the competition against Petrarca Rugby in September 1998.
Hayes, who turned 38 earlier this month, has made 94 starts in the Heineken Cup, six appearances off the bench and won two European titles (2006 and 2008).
The much-loved prop's two tournament tries came against Toulouse in the 2000 semi-final and Perpignan in a 2002 pool match.
Meanwhile, it was also a first for Peter O'Mahony at Thomond Park on Saturday night. The former Ireland Under-20 captain claimed the man-of-the-match award in his debut game in the competition.
Young backs Conor Murray and Danny Barnes made their Heineken Cup bows too, with Australian centre Will Chambers enjoying his first taste of cup action as a second half replacement.