I have a feeling it is deliberate and not related to what lounges are available to EI to buy access to. When Stobart Air had the EI Regional concession, they had lounges most everywhere there was a lounge for them to pay for. Once Emerald took over Regional, they were quite swift at removing lounge access at every Regional outstation that was not also served (and presumably paid for) by Aer Lingus mainline.

On the other hand, on the mainline network EI do make some effort to provide lounges. There are a few exceptions (I understand CDG lounge is really expensive, and they don't seem to bother with some of the bucket and spade routes), but they don't seem to make the same blanket decision Emerald/Regional made to not provide lounge access basically anywhere

A little cheekily, Aer Lingus still sell Advantage fares, which includes lounge access, on Regional routes with the same price uplift as on other routes for lounge access that literally doesn't exist.