Halfpenny, who has taken on the kicking responsibilities for the RaboDirect PRO12 side, missed a long-distance kick that would have edge Wales past France in the World Cup semi-final.
But he went some distance to making amends for that miss when he slotted a last-minute penalty having taken the kicking tee from the Scarlets' misfiring fly-half Rhys Priestland.
And the 23-year-old admitted he felt all the pressure weighing down on his shoulders when he stroked over the match-winning kick.
"It was the hardest kick I've had to take in all my life," Halfpenny said.
"As soon as it went over the emotion came - having to deal with the miss against France in the World Cup, putting that over makes it all worthwhile.
"It was end to end stuff, an exciting game. It was a tough test and all credit to Ireland, they really gave us a tough game.
"The score was back and forth and I'm just really pleased to get the result for the rest of the campaign."
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