Man-of-the-match Ferris had steam-rollered the Welshmen all evening and his double helped calm Ulster nerves which had been shredded with tries from Sean Lamont and Viliami Iongi early in the game.
After turning around 10-8 in arrears, Ulster scored 16 unanswered points aided by the boot of Ian Humphreys. They should have closed out the game there and then, but when they were reduced to 14 men with nine minutes remaining, they had to hang on after Kevin Murphy touched down to make it a converted try between the sides.
It was a catastrophic start for Ulster as Pedrie Wannenburg had a soft pass easily picked off by Lamont who casually trotted over in the corner. Scarlets nudged further ahead in the 18th minute when Fijian international Iongi swerved inside Craig Gilroy’s tackle and charged over out wide.
Humphreys narrowed the gap three minutes later with a penalty and in the 34th minute Scarlets were reduced to 14 men with their full-back Daniel Evans in the bin. With the Scarlets in disarray, Paul Marshall broke free and gathered his own kick ahead; the ball was eventually moved left for Ferris to dive over.
The Scarlets were left defending a two-point lead on the restart, but Ulster were in command, wasting a few more gilt-edged try chances. Humphreys made amends somewhat as his three penalties put a little daylight between the sides at 17-10.
Cave then delivered an exquisite pass in front of Ferris, who charged over Lamont and sidestep Evans for a classic try, which Humphreys converted.
But replacement prop Adam Macklin was yellow carded for a ruck offence, and Scarlets immediately had Ulster in reverse and number eight Murphy slipped through Ferris’s tackle to score at the posts.
But a nervous Ulster held on to claim a much-need victory.
Ulster scorers: Tries: Ferris 2 Cons: Humphreys, Pens: Humphreys 4
Yellow card: Macklin 71-80).
Scarlets: Tries: Lamont, Iongi, Murphy Cons: Jones
ULSTER: A D’Arcy; A Trimble, D Cave, N Spence (Whitten, 60), C Gilroy; I Humphreys (Jackson, 70), P Marshall (Pienaar, 63); T Court (McAllister, 54), R Best (Brady, 70), J Afoa (Macklin, 70), J Muller (capt), D Tuohy (Stevenson, 54), S Ferris, C Henry (Diack, 70), P Wannenburg;
SCARLETS: D Evans (Newton, 65); V Iongi, N Reynolds (I Evans, 38; Williams, 55)), A Warren, S Lamont; S Jones (capt), G Davies; P John (Thomas, 58), K Owens (Phillips, 55), D Manu (Gardiner, 66), L Reed, A Powell, (Welch, 55), M Gilbert (Edwards, 48), R McCusker, K Murphy.
Referee: A Rolland (IRFU).