Replacements used: Luke McGrath (UCD/Leinster) for Shanahan (53 mins), Josh van der Flier (UCD/Leinster) for Joyce (60), Rory Scannell (Dolphin/Munster) for Daly (61), Ryan Furniss (Worcester Warriors/Exile) for Taylor, John Donnan (Ballynahinch/Ulster) for McCarthy, David Panter (Corinthians/Connacht) for Scholes (all 66), Peter Dooley (Lansdowne/Leinster) for E Byrne, George McGuigan (Newcastle Falcons/Exiles) for B Byrne (both 68), McCarthy for Thornbury (73).
Replacements used: Zachery Lee for Doge (half-time), Joshua King for Rokoduru (46 mins), Penaia Cakobau for Vatubua (56), Marika Vunibaka for Yato (61), Dylan Bower for Waqa, Joeli Douglas for Seru (both 65), Aseri Robarobalevu for Ravula (69), Sailasa Kerekere for Tokalau (72).
Mike Ruddock's side needed the bonus point to keep the pressure on Australia and New Zealand at the top of Pool B, ahead of their crunch clash with the Baby Blacks on Thursday.
Rory Scholes' second try of the evening in Vannes wrapped up the bonus point by the 54th minute and prop Christopher Taylor, flanker Daniel Leavy and Darren Sweetnam also touched down in a five-try second half display.
Sweetnam's opener, allied to three Tom Daly penalties, had the Irish 14-3 in front at half-time with Emori Waqa kicking a lone penalty for the Fijians.
Three tries in the space of nine minutes, early in the second half, put the result beyond doubt as Ireland - with only four starters on board from last Wednesday's win over Australia - kept up their winning form in the north west of France.
The Fijians had nothing to lose after going down 59-6 to New Zealand in their opening game of the tournament, and a break from openside Jone Baleidau was the first glimpse of their traditional expansive style.
Scrum half Emori Waqa, their only scorer against the Kiwis, drew a kickable penalty wide before Ireland settled into their stride.
Ruddock's charges won a scrum penalty and further good work from the back row at a ruck allowed Daly boot Ireland in front after 11 minutes.
Fiji threatened at times from crisp offloads and strong broken-field runs, and their left winger Elia Ratucove was hauled down by Scholes in sight of the try-line.
But Ireland struck for their first try on the quarter hour mark. Half-backs David Shanahan and Steve Crosbie were combining well and they passed to the right for Mark Roche to send Sweetnam over in the corner.
Daly, who got the initial move going with a sidestepping run, flashed a difficult conversion attempt wide and the Carlow native pulled a subsequent penalty to the left of the target.
Fiji closed the gap to 8-3 courtesy of a well-struck Waqa penalty, but Ireland soon got further reward from their scrum with twins Edward and Bryan Byrne impressing in the front row and the loose.
Fiji held out as the Irish built from a close-in lineout, number 8 Jack O'Donoghue and Bryan Byrne being thwarted before Ireland settled for a three-pointer from Daly.
The penalties were beginning to stack up against Fiji as half-time approached, but Daly sent a 34-metre effort wide for his third miss from five kicks.
Peceli Yato failed to roll away after stopping the progress of Irish captain Conor Joyce near the Fijian 22-metre line, the resulting penalty seeing Daly get back on target.
Shanahan was to the fore when the second period got underway, finding a terrific touch on the right and it took a last-ditch tackle to force a knock on as the busy scrum half dived for the whitewash.
Ireland's scrum was becoming more and more of a weapon as they marched Fiji back towards their line. They won a succession of penalties and prop Taylor was then able to plunge over for try number two from close range.
Daly converted for 21-3 and Scholes added his name to the scoresheet three minutes later with a scintillating run from inside his own half.
The Ulster youngster broke over the halfway line, chipped over the top and darted in between two covering defenders to get the touchdown in the left corner.
With Ireland now playing with plenty of confidence and Sean McCarthy, Gavin Thornbury and O'Donoghue all prominent up front, the bonus point was swiftly registered.
A slick back-line move initiated by regular skipper Luke McGrath, who came on for Shanahan, saw Crosbie and Daly combine with the latter's lovely inside pass releasing Scholes to crash over to the right of the posts.
The winger's second unconverted effort put 28 points between the sides and the Irish management gradually unloaded the bench.
A fine touch-finding kick from one of the reserves, Rory Scannell, got Ireland into scoring range again and the Fijian defence were unlocked in ruthless fashion.
Roche scooped a long pass out for Sweetnam to shrug off a first-up tackle and tee up Leavy to slide over in the right corner.
The touchline conversion from Scannell was missed and there were groans from the try-hungry home crowd when Ireland opted for the posts from an advanced position. Scannell added the three points for a 39-3 scoreline.
With 11 minutes left, Ireland added their final try of the night and it was a brilliant individual effort from Sweetnam, the former Cork hurler who will enter his second year in the Munster Academy later this summer.
He put boot to ball near the right touchline, regathered his kick and did superbly well to stay in play, avoid a tackle and then step inside the final defender to race in behind the posts.
Scannell converted and the Irish management will have been pleased to keep Fiji try-less as Sweetnam and Roche showed their prowess in defence late on, with the Islanders unable to convert a bout of pressure into points.
Speaking afterwards, Ireland captain Conor Joyce said: "We are delighted to get the win, the Fijian boys put up a really good fight in the first half especially.
"They play a great style of rugby and are difficult to break down, but I think it was just a matter of building through the phases and eventually our foward game really took its toll on them and they couldn't really match us."
TIME LINE: 5 minutes - Fiji penalty: missed by Emori Waqa - 0-0; 11 mins - Ireland penalty: Tom Daly - 3-0; 15 mins - Ireland try: Darren Sweetnam - 8-0; conversion: missed by Tom Daly - 8-0; 23 mins - Ireland penalty: missed by Tom Daly - 8-0; 26 mins - Fiji penalty: Emori Waqa - 8-3; 30 mins - Ireland penalty: Tom Daly - 11-3; 34 mins - Ireland penalty: missed by Tom Daly - 11-3; 37 mins - Ireland penalty: Tom Daly - 14-3; Half-time - Ireland 14 Fiji 3; 45 mins - Ireland try: Christopher Taylor - 19-3; conversion: Tom Daly - 21-3; 48 mins - Ireland try: Rory Scholes - 26-3; conversion: missed by Tom Daly - 26-3; 54 mins - Ireland try: Rory Scholes - 31-3; conversion: missed by Steve Crosbie - 31-3; 64 mins - Ireland try: Daniel Leavy - 36-3; conversion: missed by Rory Scannell - 36-3; 68 mins - Ireland penalty: Rory Scannell - 39-3; 69 mins - Ireland try: Darren Sweetnam - 44-3; conversion: Rory Scannell - 46-3; Full-time - Ireland 46 Fiji 3
Referee: Shuhei Kubo (Japan)