Shelbourne striker Alannah McEvoy scored the only goal of the game for Dave Connell's side in the first of two friendly fixtures against the Italians this week.
All of the goals were scored in the first-half as the away side commanded much of the opening play and went ahead in the 27th minute as McEvoy slotted home her penalty.
After taking the lead, Ireland conceded three goals before the break to go into the second-half behind.
Ireland were much improved after the break and saw Kelly Brady make her international debut as she came on for goalscorer Alannah McEvoy in the 62nd minute.
Unfortunately, Dave Connell's side couldn't get the break-through their play deserved and they look ahead to the next international friendly against Italy on Saturday.
"We started off very well. We controlled the midfield and deservedly went ahead with Alannah's well taken penalty," said WU19 Head Coach Dave Connell.
"Then we just started giving the ball away, it was 15 minutes of madness really. They were pretty ruthless in front of goal. We made a couple of changes at the break and we were much better in the second-half.
"We're disappointed with the result of course but these are the reasons we're playing these matches, to iron out these mistakes before the elite round. Some of the performances were excellent and the players will want to put it right going into Saturday's match."
Ireland will face Italy in the second international friendly on Saturday, January 19 with kick-off at 10am (11am local time). Galway Women's midfielder Chloe Singleton is the only doubt for the game after suffering a knock in today's defeat.
Republic of Ireland: Erica Turner (GK) (Rachael Kelly 62), Rachel Baynes (Sinead Donovan 62), Doireann Fahey (Keelin McEntee 46), Roisin McGovern (Carla McManus 46), Sadhbh Doyle, Megan Mackey (C), Ciara Fowler (Katie Burdis 80), Chloe Singleton (Aoife Slattery 46), Alannah McEvoy (Kelly Brady 62), Amy Boyle Carr, Lucia Lobato.
Republic of Ireland WU19 Squad
Goalkeepers: Erica Turner (DLR Waves), Rachael Kelly (Shelbourne)
Defenders: Sadhbh Doyle (Galway Women's), Roisin McGovern (DLR Waves), Rachel Baynes (Galway Women's), Doireann Fahey (Wexford Youths), Aoife Slattery (Wexford Youths), Leah Brady (Galway Women's), Katie Burdis (DLR Waves)
Midfielders: Lucia Lobato (Galway Women's), Megan Mackey (Brighton & Hove Albion), Chloe Singleton (Galway Women's), Aoife Thompson (Galway Women's), Amy Boyle-Carr (Sion Swifts), Sinead Donovan (Galway Women's)
Forwards: Ciara Fowler (unattached), Alannah McEvoy (Shelbourne), Carla McManus (Kilkenny United), Keelin McEntee (DLR Waves), Kelly Brady (World Class)
Result - International Friendly
17/01: Italy 3-1 Republic of Ireland, Olympic Center Giulio Onesti, Rome, 1.30pm (2.30pm local time)
Fixture - International Friendly
19/01: Italy v Republic of Ireland, Olympic Center Giulio Onesti, Rome, 10am (11am local time)