Is it easy to determine the Eircode Routing Key of every address?

The Eircode Routing Key is the first three characters of an Eircode. There are 139 of these in Ireland, which is five times more detailed than Counties.  How easy is it to determine the Eircode Routing Key for an address?  How much of the address needs to be matched in order to be confident?  Surprisingly, you can easily and accurately determine the Eircode Routing Key for any address just by verifying one line of the address. 

The map below shows Eircode Routing Keys. You can see an interactive Google Map here. The areas don't follow any administrative boundaries (e.g., County boundaries) they have been designed by An Post to suit their sortation requirements for the post. Their irregular shapes may appear unintuitive, but there is an underlying logic that will become apparent later in this FAQ. 

The address line we are interested in is the last line above the County name (if any). The examples below illustrate that this is generally a Town or a Dublin Postal District (highlighted in bold)

In   Example 1 and Example 3 the Autoaddress 2.0 API will automatically match the address and return the complete Eircode, but in Example 2 Chancery is a non-unique address and we can't automatically append an Eircode. Can we return an accurate Eircode Routing Key for the address? Yes. What if it was only matched to Castlebar? Yes. What if Example 3 just had Athlone and was missing Co. Westmeath? Yes. What if Example 3 was Athlone, Co. Roscommon?  Yes.

The Eircode address database has 266 Towns. We included in our analysis every address that has one of these Town names as the last of their address above County ( even if it isn't in the Town, see this FAQ that explains Hierarchical vs Proximity addressing). This covers 70.7% of addresses, which leaves 29.3% of addresses. To include all addresses, we expanded our definition to include townlands and villages that appear as the last line of an address above the County. This increased our "Town" list from 266 to 3,802.

Then we queried all addresses that have one of these 3,802 "Towns" as their last line of address above County to see if any are found in more than one Eircode Routing Key.

Only 18 of the 3,802 "Towns" have addresses that are in more than one Eircode Routing Key area (listed at the bottom of this FAQ). The total count of buildings affected is 91,212. Thus, we can be 95.2% confident of determining the Eircode Routing Key of any given address by simply validating the last line of the address above County. The biggest issue is with Cork City which is split between T12 and T23 (with a small number in T45). These comprise 42,383 of the 91,212 buildings. Excluding Cork City, we can be 98.6% confident of the Eircode Routing Key.

This shouldn't be too surprising a result. The Eircode Routing Keys are designed around the An Post post town delivery model which sorts posts by looking at a single line of address. Incidentally, the Athlone example address we provided in the first table is actually in Co. Roscommon, not Co. Westmeath as appeared in the address. However, all of Athlone and surrounding areas are in Eircode Routing Key N37. You need to validate more of an address to determine the correct County than you do to determine the correct Eircode Routing Key even though there are five times more Routing Keys than Counties.

If the Eircode Routing Key areas suit your purpose, then you can have an easy-to-validate a solution that is five times more detailed than Counties. If you want to define your own areas for distribution, franchises, etc. then we recommend using Small Areas which can be combined to create any desired areas.  

This list comprises the 18 towns that straddle more than one Eircode Routing Key and gives the percentage of their addresses in each of those Eircode Routing Keys.