I need to download a shapefile with the boundaries (polygon) of the administrative units (localities level) for Ireland. I have searched diva-gis and osm, but they do not have data to that level.
Townlands.ie has a decent downloads page with different admin levels:
There is also the 2016 Census data which has a lot of resources:
Within the Census data is the Small Areas dataset (smallest admin level in Ireland):
Depending on what NUTS level you want, this might work: https://data.gov.ie/dataset/census-2011-boundary-files
Alternatively, you could check the European INSPIRE-Initiative for Administrative Units (Annex I).