Up until that time it had been pretty much plain sailing for the visitors who had clinched a bonus point within the hour when prop Iestyn Thomas bulldozed his way over from another dynamic rolling maul.
And when Stephen Jones added a near 50m penalty soon after to stretch the Scarlets lead to 26 points, the Irish were in danger of being humiliated. A pretty tight first half had been tipped in the Scarlets favour at 33 minutes when Dafydd James raced 30 metres to notch the first of the game's seven tries - and write his name into the Heineken Cup record book by becoming the first player to score 25 tries in the tournament.
Jones' conversion made it 10-6 to the visitors at the break. But 22 unanswered points in the third quarter saw the Scarlets race clear.
Skipper Simon Easterby, wing Mark Jones and then prop Thomas scor3ed tries and Stephen Jones took his match tally to 12 points with the boot.
Then London Irish coach Brian Smith decided to ring the changes and the introduction of Canadian No.8 Phil Murphy and French back row star Olivier Magne changed the game. Murphy charged over from the tail of a lineout in the 65th minute and two tries in the space of three minutes from centre Seilala Mapusua and Delon Armitage brought the Irish rushing back into contention.
With replacement back row man Nathan Thomas in the sin bin the Scarlets were rattled as their seemingly insurmountable lead was slashed to a mere seven points.
But they rallied round, their defence held firm and they head to Round 2 with four points in the bag.