Scarlets could only hit back through a 67th-minute try from Josh Turnbull and a last minute dive under the posts from replacement Scott Williams while George North was stretchered off to compound their woes.
Scarlets started brightly as Ulster looked sloppy in the breakdown and were penalised which allowed Aled Thomas to put the visitors into an early 3-0 lead.
Ulster had not quite clicked into their usual fluent selves and a savvy percentage game was keeping the RaboDirect PRO12 leaders plugged in their own half.
But midway through the first-half Ulster were able to turn to their formidable scrum which forced the penalty and allowed Pienaar to level the scores up.
The game was opening up into a fabulous contest as a breathless passage saw Pienaar run before possession was exchanged twice as Jonathan Davies attempted to counter before Ulster regained the ball, broke, and lost it again.
And on 32 minutes a break from the ruck by Nick Williams which took everyone by surprise led to another Ulster penalty which Pienaar tucked away and four minutes later he scored another.
Ulster made the perfect start to the second half as Diack turned it over at the lineout and following an excellent passage of play the flanker was fed by Rob Herring to score his sixth league try.
Pienaar extended the lead to 16 points in the 48th minute before Nick Williams showed his immense power to drive over - and take four Scarlets men with him - for a 57th-minute try.
The No.8 was replaced moments later to a standing ovation at Ravenhill but soon after North was downed and following an extended stoppage was removed from the field on a stretcher.
Turnbull got Scarlets' first try of the evening, showing great pace and power to make it to the line in the 67th minute.
But a fantastic line run by Trimble from the far side five minutes later saw him burst through at pace and dive straight under the posts.
The conversion was taken quickly as Ulster pushed for the bonus-point try and were awarded a penalty try as an offside Thomas intercepted a try pass.
A fine evening for Ulster ended with McComb going over the whitewash with three minutes to go and a late score by Scott Williams did nothing to dampen their spirits.
|Ulster Rugby Score Card|
|Scarlets Score Card|