The Ballymun flats themselves weren't a mistake. It was the fact that once built, they were more or less left to their own devices and consequently fell into dissrepair. There should have been more of an integrated plan. Dublin even then, was running out of space and building upwards as opposed to outwards was seen as a logical step. Even today, with land been more of a scarcity in Dublin, we still insist in building out and not providing the necessary transportation/local amenities to go with this sprawl. As a result, we now have "standstill traffic" because so many people have to commute from miles away and not by public transport, but by car. Tall buildings both resedential and office are the only way forward, if people in Dublin are to be able to afford to live there. Before long, I can see Greater Dublin spreading as far inland as Athlone, if we don't change our tune - OK maybe not as far as Athlone!
I (and I know I'll probably be the only to hold this opinion) think that knocking down Ballymun will be a big mistake.