Hugo Almeida struck twice in the first half for Bento's men, while Vieirinha and Fabio Coentrao got on the scoresheet in the second period as captain Cristiano Ronaldo made his return from knee and thigh injuries in Portugal's final friendly ahead of the FIFA World Cup.
An own goal from Richard Keogh had helped Portugal to a 3-0 half-time lead, while James McClean dragged one back for Ireland just after the break.
The big win followed Portugal's 1-0 victory over Mexico on June 6 and a scoreless draw with Greece at the end of May, which had caused much consternation in the Portuguese media.
Portugal open their World Cup campaign against Germany next Monday, and Bento argued his team had shown what they were truly capable of against Ireland.
"We're not the best team in the world and we're not as bad as they wanted to believe when we drew with Greece," the 44-year-old said.
Real Madrid star Ronaldo, who had been struggling with knee and thigh issues, played 65 minutes against Ireland in his first appearance since the UEFA Champions League final on May 24.
The Ballon d'Or holder looked strong, striking the post in the first half with a long-range strike, while one of his trademark headers led to Almeida's second goal, and Bento was thrilled with his skipper's performance.
"Yes, it was important that he played and we would not hide it," the coach added.
"A player like Ronaldo is important for any team, and that's the case for us. It's good that he is back."