Long scored 10 goals in 28 appearances for Ronald Koeman's side as they finished sixth in the Premier League, securing a Europa League berth in the process.
And the Ireland manager is "delighted" with the confidence that the 29-year-old has displayed, particularly in the latter half of the season.
"Shane's knuckled down and he's got some competition for a place at Southampton and that's been pretty healthy for him," O'Neill told reporters.
"Naturally if you want to trace it back, I think the goal against Germany in October was a massive boost. It is something special when you score the winning goal against the world champions, you would walk back to the club on a natural high.
"He's taken hold of his game, obviously got a bit of confidence from somewhere and he's trying to play how a centre forward would play.
"I am naturally delighted for him, because it will give us an extra boost."
Ireland host the Netherlands at the Aviva Stadium on May 27 before taking on Belarus in their final Euro 2016 warm-up game at Turner's Cross on May 31.