Cork City's Leah Coen Hayes, Galway Women's Annie Gough, Peamount United's Chloe Smullen, Sligo Rovers' Sarah Kiernan and Shelbourne trio Jodie McQuillan, Chloe McCarthy and Ella Maree Rice all made their first appearance for Ireland at the Belgian Football Centre.
Ireland took the lead through Erin McLaughlin in the 10th minute but goals from Imani Prez and a brace from Lyndsey Van Belle saw Belgium go into the break 3-1 ahead.
Head Coach James Scott made a number of substitutions after the break and Thomastown United's Ellen Molloy scored with her first touch in the 52nd minute.
However, Van Belle completed her hat-trick soon after and a brace from Taziana Rombauts saw Belgium win the game against a young Irish side with the second international friendly between the two sides set for Wednesday, August 14.
"We played against an excellent team today, they were very clinical and took their chances," said Head Coach James Scott.
"Looking at the positives, we had seven players make their international debut. A lot of new players in the team so it was a great learning curve for those players and it would have been the first time those players would have been involved in a game of that intensity as well.
"We got the game back to 3-2 just after the break and had a great chance to make it 3-3, but it just went wide. They came down the other end and scored but that is what happens at this level.
"It's great to be able to test ourselves against that quality opposition and I just said to the players there, that is the level we need to get to and aspire to. We've got another game on Wednesday and we'll look forward to testing ourselves against them once again."
Republic of Ireland: Leah Hayes Coen (GK), Annie Gough (Jodie McQuillan 66), Chloe Smullen, Della Doherty (C), Sarah Kiernan, Kate Slevin (Muireann Devaney 55), Ella Maree Rice (Ellen Molloy 50), Erin McLaughlin (Aoife Horgan 55 (Zoe Leonard 90)), Kerryanne Brown, Chloe McCarthy (Sarah Kiernan 50), Hazel Cawley (Rebecca Watkins 50).
Goalkeepers: Rugile Askalnyte (Bohemians), Leah Coen Hayes (Cork City) Defenders: Annie Gough (Galway WFC), Chloe Smullen (Peamount United), Jodie McQuillan (Shelbourne), Chloe McCarthy (Shelbourne), Sarah Kiernan (Sligo Rovers), Emer O’Neill (Sligo Rovers)
Midfielders: Kate Slevin (Galway WFC), Aoibheann Clancy (Limerick FC), Della Doherty (Peamount United), Ella Maree (Shelbourne), Muireann Devaney (Sligo Rovers), Ellen Molloy (Thomastown United)
Forwards: Aoife Horgan (Limerick FC), Rebecca Watkins (Peamount United), Hazel Cawley (Peamount United), Zoe Leonard (Shelbourne), Erin McLoughlin (Sea Rovers), Kerryanne Brown (Sion Swifts)
August 11: Belgium 6-2 Republic of Ireland
August 14: Belgium v Republic of Ireland, KO 10:30 (Irish Time), Belgian Football Center