This problem occupied me for several hours on Christmas day. Several google searches told me to try 1 but it still said invalid. It turned out that I was linked in to the UK itunes rather than ROI. I changed to ROI and then tried 1 as the postcode and it worked. Very frustrating this postcode business though.
Well , I was on the ROI site, and nothing worked,. I logged a call with apple support and it's a fantastic customer support, turns out the fix was to leave it blank, silly me , thanks all