Izuchukwu thought he had extended Ireland’s lead on his debut when he crossed after a scintillating Irish attack, but play was brought back by referee Hollie Davidson for a forward pass by Mack Hansen.
However, Ireland responded and Casey grabbed his first international try as he jumped over in the corner with an athletic finish from McCarthy’s pass off the back of a maul.
Prendergast added the difficult conversion before his brightest moment of his mixed first start an Ireland shirt.
The 21-year-old spotted Hansen out wide off the back of a scrum, and the his inch-perfect kick was gathered by the Connacht wing with the clock in the red.
Prendergast’s third conversion went in off the post as Ireland ended the half with a comfortable buffer.
Stockdale almost got in right at the start of the second half after a perfect flat pass by Prendergast, but Ireland showcased their dominance with Aki’s try in the 46th minute.
After more neat hands by Izuchukwu and Casey, McCarthy’s impressive debut continued as he made some yards in the 22 and unselfishly found Aki, who was able to cross unchallenged.
Stockdale, on his first Ireland start in 15 months, was forced off with an injury, which will be a worry for Ulster with their European campaign just two weeks away, and Fiji capitalised on a makeshift Irish backline when Salawa crossed for their first try of the match off a line-out, and Muntz added the conversion.
Conor Murray, playing on the wing after Stockdale’s injury, was denied a try as he gathered a pass by Ciaran Frawley, but Davidson again ruled it was forward and the score was immediately chalked off.
But McCarthy, who has just six appearances for Leinster, capped a superb Ireland debut as he powered over from a maul, which Prendergast converted, before chaos ensued as Fiji sprinted up the other end of the pitch and replacement Setareki Turagacoke crossed unchallenged.
The try-scorer was then sent to the sin-bin for a tackle on Casey, and Hansen immediately capitalised as he crossed to bring a frantic five-minute spell to a close.
Replacement Kelleher scored in the final minute to cap a strong Irish performance, and eyes will now switch to a final autumn match with the Wallabies next Saturday.