I've listened to every episode at least once. I think I might have listened to Blueprint for Armageddon 4 times now. I actually wanted to write something up on the Munster rebellion for /s/HistoryAnecdotes, but it is very difficult to find sources with a lot of information in English. There's like one guy who wrote about it, and the rebellion is only a part of what the book is about.
I remember pulling over in my car at one point in the Blueprint for Armageddon one where he's describing the British veterans vs the army of German School kids, the soldiers writing home about it. So brutal and gut wrenching.
I really liked how he described the combat and logistics of the first few weeks of the war in relation to previous conflicts. Having that context made me think about the entire war in a different way.
Absolutely love Dan Carlin's hardcore history series. I've listened to the Blueprint for Armageddon one and the one about the Lutheran rebellion. Definitely will have to check out the Celtic one.