Gareth Maule touched down from the visitors' first attack of the second half. But the Welsh region could only garner a losing bonus point as Keatley, with a 12-point tally, booted Connacht home.
A gusting wind was the main factor in a disjointed opening 40 minutes. Good work from his forwards went unrewarded when Keatley scuffed a long range penalty away to the left of the posts.
The Scarlets pack kept the ball well, facing into the elements, but lacked the territory to put Connacht under serious pressure.
The returning Lou Reed, Rob McCusker and Johnathan Edwards were hungry for work and kept forcing tackles from the hosts, for whom stand-in captain Johnny O'Connor led by example.
In the 13th minute, Murphy linked with Keatley to the right of a scrum in the Scarlets' 22. He took the return pass and dummied past Regan King and Dan Evans to score in the corner.
Keatley's missed conversion was followed by Dan Newton's opening penalty, six minutes later. The young out-half slotted over from in front of the posts after a ruck infringement from Mike McCarthy.
The remainder of the half was slugged out between the packs. There was plenty of industry but no end product.
However, in injury-time, some good harrying from Murphy and Mike McComish forced the Scarlets to cough up a penalty, and Keatley drove over from outside the 22 for an 8-3 advantage.
The second half was just 54 seconds' old when King evaded the grasp of the Connacht cover and passed for winger Maule to glide in at the left corner. Newton tagged on the tricky conversion and the game suddenly went up a gear.
The Scarlets lost their way though, failing foul of referee Andrew McMenemy at the breakdown. Keatley mopped up with two successful penalties from three attempts on goal, with the wind dying down.
On the hour mark, the Welsh side also had centre Scott Williams sin-binned for being offside at a Connacht ruck near halfway.
Connacht could not add to their try count, Michael Swift blew their best chance out wide when fumbling a pass from Troy Nathan.
Eric Elwood's men tightened their grip on the match points when Keatley added his fourth penalty, before Newton replied to give the Scarlets something to show for their efforts.
|Connacht Rugby Score Card|
|Scarlets Score Card|