After Kalaveti Ravouvou provided an early scare with a typically slick Fijian try, Ireland's well-drilled forwards took control with flanker Nick Timoney going over twice followed by Ulster team mate Robert Baloucoune.
The outcome was never in doubt when Albert Tuisue was sent off five minutes into the second half but Fiji kept in touch, even when they were down to 13 men. A Mack Hansen try had already put the often sloppy Ireland out of sight before Simione Kuruvoli grabbed the score of the game.
Ireland, much changed from last week's 19-16 win over South Africa, added a fifth try through Cian Healy.
They will welcome the likes of Johnny Sexton back for Australia next week when Andy Farrell's men look to cement their place on top of the world rankings with a 12th successive home win.