What is ironic about Brinker saying that there is a war on? Which war is he talking about and how? How does Leper's recollection of the sun's rays relate to both wars?

The war is WWII....... the irony in Brinker's statement comes with the fact he is speaking to Finny, who will never see the war and Gene, who thus far hasn't participated. For the first time, Brinker places Phinny in a situation where he has to realize that there is no future....... and eventually admit that there is a war.