The issue with that is that the CTA arrangement for passport-free travel only applies to British and Irish citizens. So while those citizens are not required to have a passport to enter either country, immigration officers do have to be satisfied that they are entitled to entry - which means that ID effectively still needs to be shown. More information here: Common Travel Area - Wikipedia, the free encyclopedia
I understand the reason, but what is odd is how going in different directions you get treated differently. Again I know the reason (which is the Irish are being stricter), just odd.