Unlike London, in Dublin, historically (and generally even now to a lesser degree) each individual part of the transport system was more important than the system as a whole. Each company views their brand and own interests as much more important than integration, they all want their own identity.
There is a Visitor Leap Card that is truly multi-modal but Dublin Bus don't promote it at all at the airport at their sales desk and instead promote their own pass that is only valid on their own services. They were requested to stop doing this and instead promote Visitor Leap, but it hasn't happened, I assume for commercial reasons.
Visitor Leap here:
https://about.leapcard.ie/leap-visitor-card