Dyson, an Englishman who won two tournaments last year, and Hanson had bogey-free rounds with four birdies each.
Gustafsson, playing with a neck injury, had two bogeys but birdied three of the last five holes. The 26-year-old Romero had eight birdies to counter four bogeys.
The four leaders took advantage of gentle conditions on the Adare Manor course, where the event is being played for the first time. After wind and heavy rain Wednesday, tees were moved forward to shorten the course by 246 yards to 7,207.
Even so, only 16 of the 156 players in the field broke par on the course, which features heavy rough.
'This feels like a major setup, like the U.S. PGA. If you don't strike it well, you have no chance,' Hanson said. 'On a course like this you want to keep double-bogeys off your card. You expect to make bogeys, but today I managed to save par three times.'
Christian Nilsson, Louis Oosthuizen and James Kingston, who bogeyed the last hole, were tied for fifth, one stroke behind the leaders.
Lee Westwood, who won last week's Andalucia Open for his first victory in 3 1/2 years, shot 71.
'I holed putts at the right time. I made a couple of good par saves and a couple birdies,' Westwood said. 'This is the Irish Open. It ought to be hard to win it.'
Paul McGinley (72) had five birdies and five bogeys, but finished three strokes better than defending champion Thomas Bjorn (75).
'I'm very pleased with that score,' McGinley said. 'The reason we Europeans don't win majors is because we don't play enough tough golf courses like this.'