Yeah the show was only ever in Standard Definition (480p)
The 1080p/720p copies are terrible upscales from streaming services.
Would recommend just grabbing the DVD copies.
I also have a 1080p upscale I made from the DVDs which personally I think is the best option https://thepiratebay.org/description.php?id=63043806.
Have you tried Zooqle? Looks like some working ones there. I find that place really good for tv. They don't have this for Derry Girls but a lot of popular shows have a dedicated page with everything laid out in seasons and it'll even have a space for upcoming eps and how long until they air (if known).