There are key differences. Ireland, Spain and Greece are members of both the EU and the Eurozone. Iceland is a member of neither.
IANAL but I understand that what Iceland did would not have been permissible under EU law. That is part of the reason why Ireland, for example, took the path it did, in relation to the bank bail-outs.
I'll give you the Eurozone...
Makes it impossible to devaluate and much harder to put capital controls in place (though Cyprius manage to do so while being in the Eurozone).
However, what Iceland did was still unilateral.
They didn't ask for anybody's permission and didn't expect their decision to be accepted.
Laws and treaties still need to be enforced and those countries are still sovereign.
What happens if a country does something that is against EU law ?
At worst, they may be expelled from the EU.
So it's really a matter of comparing the advantages of EU membership to the cost of the path of action that was taken.
But the biggest point is that Iceland has taken a very different path from Ireland, Spain or Greece with very different outcomes.
(of course, the situations were not exactly similar at the beginning but comparable enough to draw those comparisions)