I doubt it was intended to be ironic, though you're absolutely entitled to draw that inference. I think FFC wanted to reinforce Vito's humanity by showing that, in the world of violence that he inhabited, he could still feel horror for his own loss.
Then again, neither GF or II showed how Vito unleashed Luca Brasi to massacre Al Capone's gunmen in a way far more horrible than the way Sonny was massacred. That's confined to the novel.