The Dublin Postcode areas are mapped on OSM so maybe you could download them from there?
Try Geofabrik or even the QGIS plugin to do this.
I suspect they might be in line format, so they might need some work.
I plan on getting these myself in the near future so will keep you posted if I do!
I have found a very handy tool called Overpass Turbo.
This allows you to query for OSM features and export the results as GeoJSON, GPX or Raw Data.
I generated a query to return Dublin postal codes which can be found here:
I exported the result to GPX and later converted to shapefile.
This should give you what you are looking for!