Captain Sexton, playing his first competitive match in almost six months following injury and suspension, added to first-half scores from Jamison Gibson-Park, Hugo Keenan, Tahdg Beirne and Bundee Aki.
Peter O'Mahony's double and further second-half finishes from Rob Herring, the returning skipper, Joe McCarthy, Aki and Beirne sealed a resounding 82-8 victory in a one-sided Pool B opener staged in temperatures of 36 degrees Celsius.
Andy Farrell's men were given a jolt by Romania scrum-half Gabriel Rupanu crossing early on but emphatically avoided a major upset ahead of far stiffer tests against Tonga, South Africa and Scotland.
Fly-half Sexton also slotted seven of eight conversions before being withdrawn 16 minutes from time to a standing ovation and now sits just nine points shy of Ronan O'Gara's national record tally of 1,083.
The 38-year-old was back from his lengthy absence to lead his country into his fourth World Cup, while Mack Hansen replaced Robbie Henshaw on the bench in a late change following his surprise initial omission from the matchday 23.
An expectant sea of green flooded the stands of Stade de Bordeaux in sweltering south-west France. But it was Romania who made the blistering start.
Sexton's poor grubber kick was pounced upon by Hinckley Vaovasa and he raced forward to give half-back partner Rupanu a simple third-minute finish.
Ireland swiftly recovered from the sloppy opening moments to assert their authority.
Gibson-Park benefited from slick interplay involving Sexton, Aki and Garry Ringrose to dive over before Keenan crossed under the posts and flanker Beirne touched down wide on the left.
Rupanu reduced the deficit with a penalty ahead of the first drinks break, but the fourth World Cup meeting between the nations rarely threatened to be truly competitive, epitomised by an early Mexican wave.
Ireland had won all of the past clashes - nine in total - and had the bonus point in the bag in the 34th minute when the rampaging Aki claimed a fine solo score shortly after Romania full-back Marius Simionescu was sin-binned for obstruction.
Sexton got in on the act in the final moments of the half, crossing under the posts before overcoming a bang suffered in the process to slot the extras and make it 33-8 at the interval.
Ireland players had spoken all week of wanting to begin with a bang by producing a statement victory.
Undeterred by the stifling conditions, the Six Nations Grand Slam champions stretched their advantage courtesy of hooker Herring and flanker O'Mahony dotting down amid prolonged pressure.
Sexton's 17th Test try added further gloss to the scoreline before a first for 22-year-old lock McCarthy and a second of the afternoon for O'Mahony enhanced Romania's punishment.
Aki, who was sent off against Samoa in his last World Cup outing, and Beirne became the third and fourth men in green to claim a brace.
Replacement number 10 Crowley successfully added the extras from his four conversions as Ireland ruthlessly dispatched the team ranked 19th in the world to stretch their record winning run to 14 matches and satisfy the bulk of the 41,170 crowd.