If you intend to fly Ryanair or stay the night no matter when the flight is you need a visa as you do not remain airside

If flying onwards same day with Aer Lingus, assuming the connection is legal your bags should be checked through you can use flight connections

There is an immigration check at connections due to some complexity around some local Irish/UK agreements, domestic passengers etc, all you need is a passport and visa valid for destination and the boarding card