The Real Madrid stopper came on for Barcelona's Victor Valdes with just over half an hour left at Yankee Stadium and conjured up two trademark reflex saves to deny the Irish an equaliser before Juan Mata's late strike wrapped up a 2-0 win for the world and European champions.
After Roberto Soldado had put Spain ahead in the 69th minute, Casillas thrust out a hand to tip James McClean's fierce drive over the crossbar and then palmed a point-blank header from a corner on to the underside of the bar and away.
Spain now head to Brazil for this month's Confederations Cup, the only major international trophy missing from their collection, and Casillas, at 32 still relatively young for a keeper, looks likely to add to his Spanish-record 145 caps at the warm-up tournament for next year's World Cup.
Known as "Saint Iker" to adoring fans after a string of match-winning performances, he earned effusive praise from the local sports media on Wednesday.
"Casillas stops Ireland with two saves for the history books," wrote Marca, while AS added: "Two miraculous interventions from 'The Saint' Iker Casillas."
There were doubts over whether Casillas would be sharp enough to reclaim the goalkeeping spot for Spain after he fell out with former Real coach Jose Mourinho and lost his place in the starting line-up to Diego Lopez.
Mourinho, who has left Real to rejoin Chelsea, said he had benched Casillas for purely sporting reasons. He broke a bone in his hand in January and Lopez was brought in as emergency cover from Sevilla.
However, the Portuguese manager's decision followed an interview given by Casillas's girlfriend, a high-profile Spanish television presenter, in which she referred to tensions in the Real dressing room.
Lopez, a solid performer and a former Spain reserve keeper, remained Mourinho's first choice through to the end of the season as Real were knocked out in the last four of the Champions League, surrendered their La Liga title to Barca and lost to Atletico Madrid in the King's Cup final.
Seeing Casillas back to his best on Tuesday was a reminder of what Real may have missed during their frustrating season finale, when their dream of a 10th European crown and their first since 2002 again slipped away.
Del Bosque, a former Real player and coach, consistently defended his captain during his enforced absence but he was typically non-committal when asked on Tuesday which of Casillas, Valdes or Liverpool's Pepe Reina would be keeping goal at the Confederations Cup.
"He [Casillas] played well but any of the three keepers could play," Del Bosque told a news conference.
Spain begin their Confederations Cup campaign against South American champions Uruguay on June 16 in Recife and their Group B also includes Tahiti and Nigeria. Hosts Brazil are in Group A with Japan, Mexico and Italy.
