I think it was clear that the first Ryanair check in agent was incorrect here. Presumably the agent took a visual look at the item and decided it was fine, when in reality they should have referred it to the gauge. Now in many situations what should have happened is that the gate agent should have made a quick call to the check-in agent before demanding money with menaces, but in essence on LCCs it doesn't work like that, either it is within the limits or not. And I think it is fair to point out that you had a bit of responsibility to check the size before going to the airport, which was indicated in the replies around your post on 4 March above. In the end the right thing happened, albeit with some nastiness thrown in. With LCCs generally you best accept that any leeway is far from guaranteed and that it is best to measure bags to know exactly where you stand.

DUB-EDI on Aer Lingus is with Stobart and their small aircraft, which has even smaller sizes than Ryanair. But yes, they tend to be polite.