Norway, who have twice reached the final since their most recent title in 1993, looked confident throughout the game and took the lead before the 10-minute mark through midfielder Julie Blakstad.
Frida Maanum doubled the advantage three minutes later after receiving a neat pass from 2018 Ballon d'Or winner Ada Hegerberg, and Caroline Graham Hansen made it 3-0 from the penalty spot in the 31st minute.
Defender Julie Nelson, Northern Ireland's most capped player, pulled one back for the only first-timers at the 2022 tournament just after half-time, nodding home from the edge of the six-yard box.
Midfielder Guro Reiten lashed a free kick past goalkeeper Jacqueline Burns five minutes later to restore Norway's three-goal lead and inflict a heavy loss on Northern Ireland in their first match at a major tournament.
Norway moved top of Group A ahead of second-placed England, who are also on three points following their 1-0 win over Austria on Wednesday.
Norway play England on Monday in Brighton.
Nelson insisted Northern Ireland could be pleased despite their 4-1 defeat.
She told the BBC: "We knew it was going to be a difficult task. We were a bit nervy in the first 15 minutes and conceded a couple of goals, but we stepped up and did our best."
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