Ciaran Gaffney and Conor McKeon both touched down for Ireland, while the Irish were also awarded three penalty tries in conditions that did not favour running rugby.
Ireland opened their account after Munster's Dan Goggin was taken out in the air while chasing a high kick. His centre partner Gary Ringrose was on hand to slot over the subsequent penalty.
In conditions the required both teams to keep it tight, it was no surprise that Ireland's first try came from forward dominance, with Irish superiority in the scrum resulting in a penalty try.
Towards the end of the first half, Ireland pressed for their second try, this time off a scrum 15 metres out. In a fine scrummaging display that featured Munster's Rory Burke, Max Abbot and Jack O'Donoghue, two collapses resulted in the Fijian loosehead being sin binned with the referee awarding Ireland their second penalty try.
Try number three came on 49 minutes. Abbot was again on the money with his throwing, providing a good line out platform off which Ciaran Gaffney was released to cross the whitewash.
Munster also had good representation the backline. Ryan Foley made his first Irish U20 start at scrum half, Alex Wootton on his second JWC campaign lined out on the wing, while a knock to the shoulder sidelined Dan Goggin eight minutes into the second half.
O'Donoghue proved pivotal in the Irishmen's all important bonus point try. It came on 67 minutes, with the Irish captain dribbling forward from the back of another powerful scrum, and with Fiji infringing, the referee signalled for another penalty try. O'Donoghue was relieved on 72 minutes by his Munster Academy colleague Darragh Moloney.
The icing on the cake came with just two minutes left on the clock. After the Fijians spilled possession in their own 22, the opportunistic Conor McKeon scooped up possession and linked with Ringrose whose impressive footwork and neat inside pass sent the McKeon over for the closing try.
Making matters even better for the Irish, the previously unbeaten France lost top spot after going down 13-3 to Wales.
There will be a full repeat of the match on TG4 at 12.30pm with highlights being shown at 7.30pm this evening.
Speaking after the game, Head Coach Micke Ruddock was delighted with the performance;
"Is wasn't a game for fancy rugby, it was a game for taking control, and trying to chase four tries in those conditions is not easy so it was a good effort from us. I was very pleased with it, very professional".
Ireland now face England in the semi-final of the IRB Junior World Championship 2014. The game takes place on Sunday morning at 6:05am Irish time with coverage on TG4 and Sky Sports 1
Ireland U20 v Fiji
Cian Kelleher, Ciaran Garry Ringrose, Dan Goggin (Harrisson Brewer 48), Alex Wootton; Ross Byrne (Conor McKeon 69) Ryan Foley (Nick McCarthy 44); Peter Dooley (Denis Coulson 65) Max Abbott (Dylan Donnellan 57) Rory Burke (Oisin Heffernen 53); Stephen Gardiner, Ross Molony; Josh Murphy (Frank Taggart 44) Rory Moloney, Jack O'Donoghue - capt (Darragh Moloney 72)