Eircodes identify not only an address, but are more detailed, going down to identifying even a single apartment in an apartment complex, so I'm not sure if there's a searchable field for that in open street maps (one map feature should be associated with multiple eircodes). I'm a beginner in OSM editing, so maybe some more expert mappers can help you with that.

Generally, I think they'll be quite difficult to add to everything, especially because the official website ( https://www.eircode.ie/ ) has a limit of 15 searches per day, after which you need to have some commercial contract to become a partner and get the whole dataset instead ($$$). It's really something that should have been handled by the government as a public service, but as usual it got privatized by a company. There's also no consistent numbering between neighbors, so you can't really guess their eircode without looking it up (for each house/apartment...).

It would be amazing if people started adding them in OSM, but I think it'd be pretty hard to manually map a lot of them with all those those limitations...