In fact it peaked in size in 1920. But you’re broadly correct about the sense of decline post-WWI and the sense that America would be the dominant 20th-century power.
The last straw for the Irish was summary executions of the insurrectionists and the ravages of the Black and Tans -- like ICE but with arsonists and criminals, but without masks. The US will be fortunate if the parallels remain only financial.