The Irish arrival Quarantine form states exemptions
"The need to self-isolate does not apply to certain categories of passengers for example supply chain workers (such as pilots, hauliers or maritime crew members) or to passengers who are transiting en route to another jurisdiction, including Northern Ireland."
So, this implies if you transit to anywhere, then Irish self isolation is exempt.
Therefore, it is legit to fly to Dublin & get the bus to Belfast OR to take a flight to Manchester (or other UK destination) thus avoiding any self isolation.
Bonkers.