Please help if you can
Belgium birth certificates less than 100 years old will not be accessible to anyone except direct descendants.
Try requesting a "Extrait d'acte de naissance" from the Brussels website below (for some reason the English version doesn't work). You will probably need to send a copy of your birth certificate as proof of filiation.
Up until 2008, Belgium did not allow dual citizenship and you had to renounce Belgian citizenship to acquire a new citizenship. But many people did not and, as far as I know, there was no verification. The steps to regain Belgian citizenship vary greatly based on specific conditions. The Foreign Affairs website can give you more information but I recommend contacting the nearest Belgian consulate as they know more bout the subject than I do.
Don't quote me on this, as I'm not 100% sure. I think that in 1945, a woman acquired her husband's nationality at marriage and a child acquired the father's nationality. If that is correct, your mother might have been legally Irish at birth. But in either case, there should be a record of her birth in Brussels.
Have you looked into getting EU citizenship through Ireland? From what I gather, they facilitate descendants of migrant regaining their ancestors' nationality.
"Brussels" may refer to any of the 19 municipalities that it comprises. Since your grandparents married in Waterloo, it may even include some towns just south of the city. Are you able to get your grandparents' marriage certificate? It may contain information about where they resided or were stationed at the time. Your grandfather's military records may also help.
We're talking about wartime so there was a lot of movements of population. I took a quick look at the Ypres/Ieper tables and there are some VERHULST in there.