Striker Ewa Pajor opened the scoring in the fifth minute and added another goal just four minutes later in her 100th appearance for the national team.
Paulina Tomasiak made it 3-0 in the 28th minute, and Adriana Achcińska sealed the win with a goal two minutes after halftime, Polish state news agency PAP reported.
With the victory, Poland extended their lead on top of League B Group 1 to 13 points and secured a return to League A, after being relegated last year.
Poland will face Romania in their final 2025 Nations League match in the Baltic city of Gdańsk on Tuesday.
Poland are ranked 28th in the world by football's global governing body FIFA, while Northern Ireland sit at No. 44.
Later this year, manager Nina Patalon’s squad will join 15 other teams from across Europe for the Women's Euro 2025 tournament in Switzerland.
Poland head coach Nina Patalon. Photo: PAP/Adam Warżawa
Poland have been drawn into a group with Germany, Sweden and Denmark, and will open their campaign against Germany on July 4.
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Source: IAR, PAP, pzpn.pl