The four points keep Ulster right in contention for a Guinness Pro12 play-off place with six rounds still to contest, as they sit second in the table on 52 points, just behind Glasgow and ahead of Munster and Leinster – although the Scots and the Munstermen are still to play this weekend in Saturday evening’s all-important clash at the Irish Independent Park.
As Ian Humphreys ran out tonight for his 100th cap in provincial colours, three personnel changes saw Marshall take over from the suspended Stuart McCloskey at inside centre, Dan Tuohy return to the second row from injury, and Clive Ross replace Mike McComish at openside, with McComish dropping to the bench.
Elsewhere, Ulster lined up with Louis Ludik at full-back, Gilroy and Michael Allen on the wings, Darren Cave at outside centre, and scrum-half Pienaar joining Humphreys in the half-back berths.
Among the forwards, an unchanged front row triumvirate of Callum Black, Rob Herring and Wiehahn Herbst packed down in front of Tuohy and his second row partner Franco van der Merwe, while skipper Roger Wilson and Nick Williams joined Ross in the back row.
Notable among the replacements was new recruit Mike Stanley, the New Zealand-born Samoan international drafted in on a short-term contract to provide cover at out-half in the prolonged absence of Paddy Jackson.
The first half was low on spectacle, with three goal kicks from eight missed, and the only real try-scoring opportunity coming the way of the Scarlets thanks to an Ulster error.
After the visitors had done well to intercept from a sixth-minute line break by Cave, Steve Shingler just failed to open their account from wide on the Ulster 10-metre line when Williams was penalised for holding on to the ball too long in the tackle. The out-half had a second bite at the cherry moments later, however, and this time hit the target from a less acute angle.
Missed penalty attempts from Pienaar on 14 minutes then Shingler again five minutes later did little to kickstart a somewhat sluggish start to the game, and indeed the Kingspan crowd had to wait until the 25th minute to herald the first home points, dispatched from in front of the posts by Pienaar.
Shingler added a second successful penalty just before the half-hour mark but, as the Scarlets played themselves into trouble with a hesitant clearance of the Ulster restart, flanker James Davies infringed at the breakdown to present Pienaar with three easy points.
Then a poor pass from Humphreys to Cave, as Ulster moved into a promising break with men over, gifted possession to Scarlets Number Eight John Barclay, and the Scot ran over unopposed from the left wing for the simplest of tries on 36 minutes, converted by Shingler.
Half-Time Score Ulster 6 Scarlets 13
It took just five minutes of the second period for Gilroy to consign the disappointment of the first half to memory, as the winger picked up a Scarlets clearance from deep and proceeded to slalom his way through half-a-dozen attempted challenges for an outstanding individual try.
Although the Pienaar conversion looped wide, a rejuvenated Ulster wrenched possession back soon after the restart thanks to alert work from Herring and, as play spread from right to left, Humphreys went some way to atoning for his earlier mistake with an exceptional flipped pass to Marshall, outfoxing the Welsh defence to such an extent that the centre enjoyed a virtually clear path to the whitewash.
Pienaar converted and, with Ulster chances now coming thick and fast, the scrum-half’s 53rd minute combination with the lively Allen almost produced try number three, but for a desperate tackle bringing him down a metre from the line.
The Springbok had only a matter of minutes to wait before his next chance however, as Williams scooped the ball into his hands out of the back of a dominant Ulster scrum, and he dashed over for a well-deserved score, to which he added the conversion.
To their credit, the Scarlets spent the next 10 minutes of the tie pushing and probing in Ulster territory, but staunch defence kept their advances at bay and, with Stanley making his bow on 72 minutes, the stage seemed to be set for a grandstand finish with the potential of a bonus point.
It wasn’t to be, however, as Ulster relinquished possession in the middle of the park several times over, and indeed Scarlets replacement Kristian Phillips capitalised on a quick restart to ground in the corner on 78 minutes, Shingler adding the extras with the last kick of the match.
Next up for Ulster are two more matches against Welsh opposition, away to Newport Gwent Dragons on Sunday 8 March before the visit of Cardiff Blues on Friday 27 March.
Full-Time Score Ulster 25 Scarlets 20
Ulster (15-9)
L Ludik; C Gilroy, D Cave, L Marshall, M Allen; I Humphreys, R Pienaar; (1-8): C Black, R Herring, W Herbst, D Tuohy, F van der Merwe, R Wilson (c), C Ross, N Williams;
Replacements (16-23)
J Andrew, A Warwick, B Ross, L Stevenson, M McComish, P Marshall, M Stanley, R Scholes.
Scarlets (15-9):
J Williams; H Robinson, R King, H Parkes, M Tagicakibau; S Shingler, A Davies; (1-8): P John, K Owens, P Edwards, G Earle, L Rawlins, A Shingler, J Davies, J Barclay;
Replacements (16-23):
R Elias, R Thomas, B Leung, J Payne, R McCusker, G Davies, A Warren, K Phillips