unable to use payment features because invalid postcode.

Make sure the country selected for the Apple ID is your country.

Postcodes are country specific, and if your Apple ID is using a different country the postcode requirements can be different, and will not work with postcodes or eircodes from a different country.

The sign in locations provided by the email notifications when you sign in to your account, can be inaccurate to your actual location. As rather than identifying your own location, they use IP address geo location.

The IP address comes from your ISP, or mobile / cellular service company and is normally geo located to their server's location. So in some cases it can be somewhat close by and in other cases it can be very far away from you.

If you are in Ireland then Dublin is a reasonable location for the ISP to be in.

See here: Change your Apple ID country or region - Apple Support