Dn3 may be logical, but its meaning is not immediately obvious. In the rest of the Republic we don't use post codes at all, and I don't believe most people would see Dn3 and straightaway think, Ah...Dublin. Every letter I've ever seen or written to the capital, or address advertised, will just say "Dublin 1" (or whichever) - that is, when they bother with a number at all. The abbreviation is not in common usage even locally. If you include the district name (Clontarf, Ranelagh, Rathfarnham etc) you really don't need a number as well. (This is bound to change as the housing explodes outwards, but the old long-established areas of the city are well recognised.) If you write "Dn" even a lot of people here won't understand what you're talking about. We just call it Dublin.

And though the Irish name for the city may be known here, probably a lot of visitors from Britain and America and elsewhere will not make the connection because there's no obvious association between the words (as there is with, say, Cork = Corcaigh).

I think the point Eileen is trying to make is that head office is trying to do everything they can to obscure their identity. This is certainly an effective way to do it. "Dn3" is largely meaningless if you don't automatically think Post Code - which people here don't, because outside the capital we don't have them.

And how on earth do you SAY it? That would lose everyone. Which I suspect is the whole point.