David Silva and substitute Cesc Fabregas also scored second-half goals as the defending champions improved to four points in Group C, the same as co-leader Croatia.
Spain dominated the tempo and scoring opportunities through its quick combinations, taking 27 shots and putting the team on course for a spot in the quarterfinals.
Torres, who scored in the Euro 2008 final, put Spain ahead early after dispossessing Richard Dunne inside the area and scurrying past Stephen Ward before sending an angled shot over goalkeeper Shay Given in the fourth minute.
"We wanted to win, to make a better impression than we did against Italy," said Torres, who became Spain's third all-time leading scorer with 30 goals in 95 appearances. "The performance of the team was really good. It was great."
The goal marked the start of Spain's assault on the Irish, who had started well with Simon Cox forcing Spain goalkeeper Iker Casillas into a tricky save in only the second minute.
But Spain's combination passing got better, leaving Ireland in a constant chase. Spain midfielders Xavi Hernandez and Xabi Alonso exploited every crack through the middle with pinpoint passes that were either spoiled by poor finishing, blocked shots or saves.
"For the vast majority of time, we were chasing shadows," Ireland midfielder Keith Andrews said.
Silva was a continuous threat inside the area before finally converting early in the second half. The Manchester City forward gathered the ball inside the area in the 49th before showing some patience by rolling a left-footed shot around two defenders and through Dunne's legs and out of the reach of Given.
Given still made clutch stops on shots from Andres Iniesta, Xavi and Alonso, who pushed forward alongside fullbacks Alvaro Arbeloa and Jordi Alba as Spain looked for goals.
While Ireland enjoyed a brief spell of chances in the second half, with Casillas stretching to save a shot from Robbie Keane in the 75th, it was Torres who showed his late season turn in form at Chelsea would continue at Euro 2012. Torres ran on to Silva's pass and held off Dunne's pressure to shoot around Given in the 70th.
"That why we put him on, to do what he did today, to score goals," Spain coach Vicente del Bosque said. "It was an exemplary game for us in many senses. We played a very complete game.
Fabregas, who scored against Italy while playing in an unlikely attacking role, received Silva's corner at the near side and sent a shot off the inside of the far post to complete the rout in the 83rd.
Ireland will play its final match against Italy, which has two points and needs of a victory to advance to the quarterfinals. Spain can advance with a draw against Croatia on Monday.
"To go out looking for a draw would be very dangerous," Del Bosque said.
