The PSTN would be a non-exhaustive list of public *sector* schemes. From your original comment, you're looking to check public *service* schemes.
I think the Statutory Instruments under the Pensions Act would be a place to look for public service schemes (because - as far as I know - only public service schemes are exempt from the funding standard).
To my knowledge, An Post are a public sector organisation rather than public service. If you're looking to see if recent An Post service would result in someone joining a pre-existing public service scheme rather than the Single Scheme, it does not - it must be a public service scheme specifically.