I'm landing in Dublin via American Airlines at 5AM on a Friday in June. I hope to catch a Ryanair flight at 9AM to Kerry. Assuming things are on time (this is a big assumption, I know), is 4 hours enough time to gather luggage and get through passport control/immigration?
Four hours should be sufficient.
I arrived in Dublin a few weeks ago, on United. We landed, I deplaned, went through passport/ immigration, retrieved my checked baggage and was outside the terminal in less than 45 minutes. And I flew economy, so my bags weren’t expedited. Four hours should give you time to walk a bit, get some coffee and scones. Maybe a bit early for a Guinness?
Dublin airport is not huge, so getting from your arrivals gate to your connecting flight won’t take long.
American Airlines arrives at Terminal 2 and Ryanair operates out of Terminal 1. It's just a few minutes walk between the terminals.