There is plenty of Toomey (+ variants) info on irishgenealogy.ie, but without knowing John Toomey's parents names (which you should get from his Civil & Church marriages to Margaret Carroll), it's not possible to find out who the brother who died young is. Unless you already know his name & age of death??
It seems to me you will have to risk buying the cert for the John Toomey who is buried in Glasnevin, from which you should be able to ascertain if he is "your" John. You could then get his Death Cert which will give cause of death.
Cheers & good luck.