The host nation provided the hearty crowd in attendance at United Park in Westport with a festival of goals despite miserable wet conditions.
Sharon Boyle's team wasted no time in getting on the front foot and found themselves a goal ahead after only five minutes when Nadine Clare controlled the ball well inside the box before slotting it into the net.
Not content with the one goal, the host side struck again three minutes later when Aoife Slattery's screamer nestled itself in the back of the Northern Ireland net.
Louise Masterson added another in the 26th minute after a sloppy ball from Northern Ireland keeper Jade McPhee landed at her feet. The winger cool accepted the gift before lobbing McPhee and making it 3-0.
Northern Ireland were found flat-footed on too many occasions against their slick neighbours and they could have been punished more heavily only for the conditions
Amy Boyle Carr added one more before the interval by rolling the ball under McPhee after collecting a clever through ball from Sinead Donovan.
Northern Ireland needed a good start to the second half but it was their opponent's substitutes that set down a marker.
Emily Whelan exerted immediate influence on the game and she will have considered herself unlucky not to have bagged a few early goals.
Ireland eventually added to their tally in the 57th minute when Lucia Lobato reacted quickest to a fine save by McPhee before Whelan put a gloss on the result with seven minutes remaining by sliding the ball into the corner of the net.
Next up for the youngsters is Lithuania in Castlebar on Wednesday before they face a stern test against Denmark on Friday back in Westport.
Republic of Ireland: Rachel Kelly (Karen Connolly 45), Nicole Smyth (Aoife Brophy 45), Kayla Brady (Kate McClenaghan 45), Aoife Lynagh, Leah Brady (Oleta Griffin 61), Nadine Clare (Mia Dodd 61), Aoife Slattery, Sinead Donovan, Lucia Lobato (Isibeal Atkinson 72), Amy Boyle Carr (Emily Whelan 45) Louise Masterson (Zara Foley 38)
Northern Ireland: Jade McPhee (Maddy Harvey Clifford 80), Toni-Leigh Finnegan, Kirsty Cameron, Rachel McLaren, Beth Chambers (Saskia Love 67), Kerry Beattie (Jessica Really 67), Rebecca McKenna (Katie Stanfield 67), Aimee Lee Peachy (Eirinn Canavan 45) Sadie Craig (Joely Andrews 45), Carla Devine, Casey Howe (Leah McCabe 80).
Referee: Richard Barry.