The 28-year-old Leicester defender fired home from close range after substitute keeper Iker Casillas had turned Stephen Kelly's 81st-minute header on to the crossbar to spark mass celebrations at Yankee Stadium in New York, but his joy proved to be shortlived as an assistant referee's flag handed the Spaniards a reprieve and they won 2-0.
St Ledger said: "I went away celebrating, but it was actually a really good decision from the linesman, having seen it again. But it would have been a good feeling, obviously. I think Coxy (Simon Cox) was offside from Stephen Kelly's header. I don't know if that's right, I'm not sure."
He added: "Obviously, Casillas touched it on to the bar. I don't know the rules these days, they keep changing and it's open to interpretation. On another day, you might have got it."
St Ledger's big moment came as Ireland launched a late assault in a bid to cancel out substitute Roberto Soldado's 69th-minute opener, which had finally broken the deadlock.
Up until that point, Giovanni Trapattoni's men had rediscovered a measure of the dogged resilience which had served them so well in the past to frustrate a side which had trounced them 4-0 at last summer's Euro 2012 finals.
Cruelly, Spain made sure of the win with two minutes remaining when Chelsea's Juan Mata added a second which was justified on the balance of play, but harsh on Ireland in the circumstances.
St Ledger said: "To be fair, I thought we actually did a lot better than we did in Poland. They are an unbelievable side for hanging on to the ball - but they do that to nearly every team in the world.
"We watched highlights of the France game and the possession stats were probably nearly just the same as here, so it's not just us. They are a fantastic side and I can't see any other winners of the World Cup next year."
Despite his disappointment at the manner of defeat, Trapattoni conceded that Spain deserved to win, and he said: "The better team won - they are the champions of the world. They controlled the game for 45 minutes, but we had a good balance and a good personality."
- Sports & Recreation
- Iker Casillas
- Giovanni Trapattoni