1. As a US citizen you can’t use social security as income as the US embassy wont issue an income affidavit.
For the first year visa and first extension you’d need 800k in a Thai bank.
After that you could switch to the 65k per month international transfer method.
A Non OA doesn’t require funds in a Thai bank for the initial visa but it requires (as do subsequent extensions) mandatory health insurance that some over 70 struggle to get. A Non O doesn’t require that.
Getting a Thai bank account is a struggle these days. An agent is probably the way to go.
The application is all done online. You are correct that the London embassy handles applications from Ireland but you don’t visit the embassy at all (you’re not even allowed to these days).
As to proving that you’re in Ireland, that may be harder, as you won’t have any entry stamps to prove that you are, but someone living in Ireland wouldn’t either. I believe most just upload copies of their passports and something like a driver’s license or utility bill and the like.
Send them what information/documentation you have. If they want more they’ll ask for it.