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Fishlock, 30, became the first Wales international to win 100 caps.
The Seattle Reign midfielder marked the occasion with an unstoppable strike on 39 minutes to send Wales 2-1 up.
Georgia Evans and Natasha Harding got Wales' other goals while Rachel Furness scored from the spot for Northern Ireland after Kayleigh Green handled.
It was also a special day for Notts County Ladies midfielder Angharad James who won her 50th Wales cap.
Wales are playing Northern Ireland twice in two days in preparation for the upcoming Fifa Women's World Cup Qualifying campaign.
Qualification begins in September for the tournament in France in 2019, with the draw taking place in April.
Wales and Northern Ireland meet again at Ystrad Mynach on Friday, 7 April.
Jess Fishlock celebrated her 100th cap in style as she helped Wales to a comfortable win over Northern Ireland with a terrific first-half goal.