Iain Henderson made his presence felt on his return
Despite an excellent first half performance in Llanelli this evening Ulster were edged out late by the Scarlets, who consolidated their top four spot in the Guinness Pro12 thanks to the win.
The Welsh outfit sit fourth on 40 points while the Ulstermen's losing bonus point sees them remain in sixth place on 33 points.
A Jacob Stockdale try and eight points from Paddy Jackson's boot had Ulster in the driving seat at Parc y Scarlets until the sixty-minute mark when a contentious penalty try swung the advantage in the hosts' favour.
Notable also in the match was the impact of World Rugby's new directives for the punishment of high tackles, which saw a player from each team yellow-carded for making contact above the shoulders - in the case of Ulster, Sean Reidy in the penalty try incident which effectively decided the game.
Ulster's cause had been bolstered by the return of the four Ireland internationals who had been rested under the terms of the Ireland Player Management Scheme for last weekend's disappointing defeat to Leinster.
Hooker Rory Best, lock Iain Henderson, out-half Jackson and centre Luke Marshall all came straight back into the starting XV - but a litany of injuries sustained in the New Year's Eve fixture meant five further changes to the team.
In the backline, Louis Ludik came in at full-back for Charles Piutau while Stockdale replaced Tommy Bowe on the wing, the Monaghan man moving to the bench. Paul Marshall deputised for the injured Ruan Pienaar at scum-half, while winger and captain Andrew Trimble and centre Stuart McCloskey retained their places from the game at the RDS.
Multiple changes up front brought in a new prop pairing of Callum Black and Ross Kane either side of Best, while Kieran Treadwell continued at lock alongside Henderson. Finally, an injury to Roger Wilson saw Sean Reidy shift to Number Eight, with Chris Henry coming in at openside and Clive Ross retaining the number six shirt.
There was plenty to praise about Ulster's first half performance, with well-organised defence, good discipline, careful handling in difficult conditions and efficient use of the few chances that came their way.
Under heavy rain Jackson did well to atone for an early charge-down of his kick from Dan Jones by catching up with his opposite number as he raced after the ball and doing just enough to ensure there was no touchdown as the two slid for the line.
Jones got his name on the scoresheet on 10 and 14 minutes courtesy of two penalties, Jackson clawing back three points straight away with a kick from 45 metres after a high tackle on Ludik. Then a rolling maul on the Scarlets '22' paid dividends as quick thinking from Paul Marshall allowed McCloskey to bludgeon through the Welsh defence line, Stockdale picking up to run over his third try of the season.
Jackson's conversion led to a good 10 minutes of Scarlets pressure, which Ulster defended manfully until an eventual knock-on allowed Stockdale to clear his lines.
Then good pressing high up the park from Ulster won a lineout on the Scarlets 10-metre line, which allowed the visitors to close out the half going through phases until the award of a penalty in the dying seconds saw Jackson extend the score to a converted try.
Half-Time Score Scarlets 6 Ulster 13
More of the same from Ulster after the break kept the Scarlets pinned back in their own half for 10 minutes until Best was pulled up by referee Marius Mitrea for not releasing in the tackle, and as the Scarlets jolted into life an offside from Kane gave Jones his third penalty of the contest on 53 minutes.
Changes at prop and scrum-half brought in Andy Warwick, Jonny Simpson and David Shanahan just before the hour mark, but the Scarlets recovery continued and when Aled Davies peeled off the back of a rolling maul, a tackle around the neck from Reidy as the replacement scrum-half dove for the line resulted - harshly perhaps - in the penalty try and a stay in the sinbin for the Number Eight.
Jake Ball was soon to join Reidy off the pitch for a similar challenge on Ross - again a disputable decision - and with parity of numbers restored Ulster wasted little time in pushing on to reclaim their lead, Jackson coming closest with a diagonal break on 67 minutes.
However the breakthrough would not come and, with both teams back to their full complement, the Scarlets held on to the ball - and their lead - for a fine comeback victory.
Full-Time Score Scarlets 16 Ulster 13
Scarlets (15 - 9) Johnny McNicholl; Liam Williams, Jonathan Davies, Hadleigh Parkes, Stef Evans; Dan Jones, Gareth Davies
(1 - 8) Rob Evans, Ken Owens (c), Werner Kruger, Jake Ball, Tom Price, Lewis Rawlins, James Davies, John Barclay
Replacements (16 - 23) Ryan Elias, Wyn Jones, Nicky Thomas, Joshua Macleod, Will Boyde, Aled Davies, Aled Thomas, Steffan Hughes
Ulster (15 - 9) Louis Ludik; Andrew Trimble (c), Luke Marshall, Stuart McCloskey, Jacob Stockdale; Paddy Jackson, Paul Marshall
(1 - 8) Callum Black, Rory Best, Ross Kane, Kieran Treadwell, Iain Henderson, Clive Ross, Chris Henry, Sean Reidy
Replacements (16 - 23) John Andrew, Andy Warwick, Jonny Simpson, Franco van der Merwe, Pete Browne, David Shanahan, Brett Herron, Tommy Bowe