Eve Higgins scored the pivotal try as Ireland launched their Women's Rugby World Cup campaign with a 42-14 victory over Japan in Northampton on Sunday.
Japan were on the attack and pressing for another try, at 28-14 down, before inside centre Higgins picked off a floating pass and ran the length of the field to score.
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Her try left Japan with too much of a mountain to climb in this Pool C clash after Ireland wing Amee-Leigh Costigan's fifth-minute try had broken the deadlock.
Ireland's physical superiority was rewarded by a maul try from Neve Jones before Japan, playing well with ball in hand, deservedly got on the scoreboard with a close-range score from Haruka Hirotsu.
Ireland wings Costigan and Beibhinn Parsons combined expertly for a fine try before Fiona Tuite was the willing recipient of Higgins' slick footwork in midfield.
Japan's maul made an impact in the second half, paving the way for a try from flanker Masami Kawamura, before Higgins' interception put the result beyond doubt.
Ireland, who finished third in this season's Women's Six Nations behind champions England and fellow European heavyweights France, added another try late on through replacement Enya Breen.
"Games are never perfect," Ireland coach Scott Bemand told the BBC
"Getting off to a fast start was an important part of what we worked on.
"We were slow out of the blocks in the second half so getting the bonus point before half-time was quite crucial really for a young team, 17 caps average."
Japan coach Lesley McKenzie, meanwhile, was proud of the way her side stuck to their attacking game.
"They were outstanding," she said.
"We didn't get the result that we wanted but we wanted the performance because it was going to captivate people, it was going to engage people and they are going to think what a phenomenal style of rugby these girls play."
In Pool D, South Africa won their first Women's World Cup match since 2010 by thumping tournament debutants Brazil 66-6 in Northampton, with Springbok Number Eight Aseza Hele scoring three of their 10 tries.
The power of Hele, formerly of Premiership Women's Rugby side Harlequins, proved too much for Brazil, the lowest-ranked side among the 16 competing teams.
Brazil, the first South American team to feature at a Women's Rugby World Cup, their got points on the scoreboard through two penalties from fly-half Raquel Kochhann.
South Africa are now set for a clash with Italy, beaten 24-0 by France on Saturday, with Springboks coach Swys de Bruin insisting his side must raise their game if they are to secure a win that will likely book their place in the quarter-finals.
"We know we made too many errors if we are to get a result against Italy," he said.
"To be realistic, we're ranked 12th and Brazil 16th; we are playing number four (France) and seven (Italy) now."
He added: "What pleased me last night was that no one got a bonus point, France or Italy, that's positive for us. We need to somehow pull off a victory against Italy, but we need to play far better."
