However, Ulster made hard work of dispatching the Welsh visitors after pushing out a 21-3 lead after 30 minutes thanks to those two Jackson efforts and another from Tommy Bowe.
But after firing blanks for most of the second half, it was replacement prop Tom Court who pilfered the fourth try to take Ulster within three points of second-placed Munster.
Ferris made his introduction after 62 minutes to athunderous reception and got through 20 minutes of hard graft.
Having scored two converted tries, Ulster were showing plenty of pace when their backs got into gear, but likewise the Scarlets, with Rory Scholes denying Kristian Phillips before Gareth Davies scored a fine try.
Having gone in at the break 21-10 to the good, Ulster had to produce some stout defence to keep the visitors at bay — two missed long-range efforts by Pienaar didn’t help.
However, the return of Ferris and Court’s effort finally made sure of the win!
Scorers for Ulster: Tries: P Jackson 2, Bowe, Court. Cons: Pienaar 3.
Scorers for Scarlets: Try: G Davies. Con: Barclay. Pens: Barclay, Barkley.
Payne, Bowe, Cave, McCloskey, Scholes, P. Jackson, Pienaar, Black, Herring, Afoa, Muller, Diack, Wilson, Doyle, Williams.
Replacements: Gilroy for Bowe (63), R Andrew for Scholes (74), Heaney for Pienaar (77), Annett for Herring (48), Lutton for Afoa (70), Stevenson for Diack (74), Ferris for Williams (63).
J Williams, K Phillips, Maule, Barkley, Climo, A Thomas, G Davies, John, Myhill, Adriaanse, Earle, Snyman, Turnbull, Barclay, McCusker.
Replacements: Lewis for Maule (70), R Evans for Climo (65), A Warren for John (65), Kelly for Earle (74), Timani for McCusker (47).
Referee: Neil Hennessy (Wales).