The essence of Bono's answer to the tax question seems to boil down to: "'U2' functions as a large corporation, and therefore it's entitled to act in its own economic interests, regardless of whatever I say or we sing about, etc.".
It's basically the same thing The Pope would say if anybody asked him why The Vatican hordes massive stores of gold and money, hidden from the poor and devout behind locked doors. I guess the problem I have with Bono's answer is that any leader of a nation-state could basically say this as well: "The German state functions as a corporation and must remain solvent in the protection of the German people; therefore, we cannot afford to contribute to world aid." In other words, his own defense kind of undermines his own goals for activism.
Anyway, what's really interesting to me is how U2 is called to task for these things, while no other stadium filling act in rock history has been. The clear message from the mass public is: "If you are going to be a do-gooder, prepare to be bitch-slapped for every tiny fault we can find with your way of life." Which is a bit sad, but I guess it's just human nature.
I also think the topic wasn't necessary in the context of this interview, but in a way I admire Gay Byrne for asking it -- simply because it means the interview wasn't a total butt-kissing enterprise.