Captain Barry Coffey gave Jim Crawford's side an early lead in the Pinatar Arena when his shot defeated Belgian goalkeeper Maxime Delanghe.
In the first of two penalties, Laurens Symons scored for the Belgians of the half-hour to ensure the teams went into the break level.
After the break, Tyriek Wright scored from the spot beating Delanghe to ensure Crawford's side won.
Speaking after the win, Under-18 Head Coach Jim Crawford said his side were full value for the win.
"We created plenty of chances and defended really well. The players done themselves and their country proud, with a performance against one of Europe's best.
"Our first half performance was so good, I was disappointed we didn't go into the break in the lead. In the second half, we held on to the lead. The boys defended fantastically."
The team return to action on Saturday afternoon against England.
Belgium: Maxime Delanghe, Richard Makutungu, Lars Dendoncker, Wout Coomans, Pierre Diedhiou, Milan De May, Adnan Ugur, Dylan Mbayo, Jamie Yayi Mpie, Evangelos Patoilidis, Laurens Symons
Republic of Ireland: Kian Clarke; Max Murphy, Kameron Ledwidge, Oisin McEntee, Reece Staunton; Tyriek Wright (Adam Idan 76), Barry Coffey (Jason Knight 46), Luca Connell, Callum Thompson (Marc Walsh 65); Sean Brennan (Conor Grant 87), Jake Ellis (Shane Flynn 46).
Referee: Jose Joaquin Gallego Gambin
Photo courtesy of Pinatar Arena