So say your killing the ender dragon okay. You take out the end crystals first, and when you take them out they do no damage to the surrounding blocks. Including the iron bars.

When getting rid of the end crystal, the iron bars around the crystal do not break when the crystal blows up.

All you have to do is have the iron bars spawn naturally and then remove one iron bar and blow up the crystal and then you'll see that the iron bars are unaffected.