Nigel Hasselbaink opened the scoring before Stevie May netted a deadly brace, one from the spot.
Hasselbaink then flashed a header straight at Samson from a whipped May cross but the Dutchman would atone for that miss moments later.
Wotherspoon split the defence with an incisive 30-yard pass to the in-form striker who steadied himself before slotting past Samson to break the deadlock.
Sharp-shooter May then effectively the game as a contest just after the half-hour.
Gary Miller fed an inviting ball forward but Mark O'Hara should comfortably have dealt with it. Instead the 17-year-old stood off and May gathered possession before sliding the ball past the helpless Samson from 18 yards.
The confidence then visibly drained from the Ayrshire side who were a par below the hosts at every level.
May could and should have wrapped up proceedings minutes into the second period as the Killie defence parted like the Red Sea.
But the man of the moment failed to beat Samson after being re-fed by strike partner Hasselbaink from six yards.
Not that it mattered. On-loan Celtic youngster Irvine walked after going through the back of Chris Millar. It was the Australian's second booking after harshly being cautioned by Willie Collum in the first half for diving.
Saints secured the easiest of wins when Jeroen Tesselaar recklessly hauled down Millar with May sending Samson the wrong way from the resultant penalty.
The away fans vented their ire at manager Allan Johnston but there was no way back for Killie.
Clohessy did reduce the deficit when his cross drifted over Alan Mannus with 22 minutes left.
Barr was then sent off with six minutes remaining after felling May on the angle of the box to compound a woeful day for Kilmarnock.
- Sports & Recreation
- Craig Samson
- Nigel Hasselbaink
- Sean Clohessy
- David Wotherspoon
- Jackson Irvine