postal code field is required on billing details forms

The "postal code" field on the billing details section of event registration and contribution forms is a required field. I need to make this field optional as Ireland does not have any zip / post codes.

I've tried doing it in a hook_civicrm_buildForm() but the billing address fields don't appear in the list of form elements or the elementIndex, so I can't figure out a way to do it short of hacking CiviCRM itself which I'd prefer not to do.

Can we make the "required-ness" of this field configurable per country? or at the very least optional?