But Ulster recovered and scored 24 unanswered points through two Ferris tries and the boot of fly-half Ian Humphreys to inflict a first defeat for Scarlets in nine games in all competitions, despite a late Kieran Murphy try.
Ulster got off to the worst possible start when straight from the kick-off the returning Ferris passed the ball straight to wing Lamont who couldn't believe his luck before running in for an easy try in the left corner.
The wind was playing havoc with any kicking attempts and Stephen Jones missed his conversion before Humphreys hit the post with a penalty attempt as Ulster looked to get back into the game.
And they were made to rue that miss opportunity as on 19 minutes Scarlets' Tongan international Iongi demonstrated speed and power to cross in the right corner, Jones wayward again with his conversion attempt.
Just two minutes later Humphreys did get the Ravenhill faithful off their feet as he reduced the Scarlets lead to just seven points with a penalty.
The Scarlets were reduced to 14 men just after the half-hour mark when full-back Dan Evans was sent to the sin-bin for deliberate offside, and they were made to pay for it just two minutes later as Ulster grabbed their first try of the game.
Starting in their own half the ball found its way to scrum-half Paul Marshall who gathered his own chip forward before Ulster worked the ball from right to left for Ferris to atone for his earlier mistake and barge his way over for a try, Humphreys wide with his extras attempt.
Ulster clearly had their backs up after finishing the first-half strongly and went ahead just two minutes after the half-time break through a Humphreys penalty before stretching their lead moments later with another three-pointer.
The Ulster fly-half was at it again just before the hour mark with a booming penalty and then converted Ferris' second try of the game five minutes later to give his side a comfortable 14 point lead.
Scarlets however didn't go down without a fight and after Ulster replacement Adam Macklin was shown a yellow card, the Irish side conceded penalty after penalty and No.8 Murphy forced his way over for their third try of the game, although it was too little too late for the Welsh side.
|Ulster Rugby Score Card|
|Scarlets Score Card|