23 postal districts and my postcode used to be D15 (which I shared with about 50,000 people). Recently the government has launched a system called eircode which provides each address in Ireland with a unique post code. I thought this was good news until I found out the detail of how the new codes work.
My new Eircode is D15NY1* (I obscured the final character to avoid publishing my address). The D15 portion is the same as before and shows roughly where I live, but the NY1* portion is simply a random sequence of characters with no meaning. For example my next door neighbours' post codes are D15KD9* and D15WN8* so they share nothing in common apart from the D15.
For some bizarre reason the postal service think it would be a privacy issue if your post code were to reveal where you live (is this not the whole point of a post code?). A more likely explanation for the choice of random characters is the fact that the promoters of this system are selling a database which maps between post codes and geographic coordinates. I guess commercial users might want to buy the database, for casual use there is a web site to do lookups.