The Lionesses, who finish top of Group A, will go into the last eight having not conceded a single goal and with three wins from three games.
Tournament top scorer Beth Mead extended her tally to five goals, while substitute Alessia Russo scored two goals in five minutes after the break.
Northern Ireland, who were competing in their first women’s major tournament, finish bottom of the group without a win.
It was not a home nations derby with much on the line but England were heavy favourites and were expected to demonstrate why in Southampton.
With manager Sarina Wiegman absent after testing positive for Covid on Friday, England were frustrated in the early stages at St Mary’s Stadium but soon imposed their quality.
Chelsea’s Fran Kirby produced a piece of magic to break the deadlock when she expertly curled the ball into the top corner, before Mead precisely drilled in a low strike to make it 2-0 before half-time.
Russo came off the bench to score for the second consecutive game, nodding in Mead’s chipped cross, before spinning away and slotting in a second goal in five minutes.
An own goal from Northern Ireland’s Kelsie Burrows, who flicked Mead’s cross over goalkeeper Jackie Burns, completed the rout.
Northern Ireland struggled to compete after the break but had shown glimpses of threat in the first half – Lauren Wade tested goalkeeper Mary Earps from close range, while Rebecca Holloway played some teasing through balls.
Kenny Shiels’ side leave the tournament with plenty of lessons to learn, but their fans were in celebratory mood as they chanted, banged drums and stood for almost the entirety of their final game at Euro 2022.
For England, a quarter-final against Spain or Denmark awaits next Wednesday, and they strengthened their status as a team few will want to face en route to the final.