As far as I am aware, pre-clearance is only for entry to the USA, and only on pre arranged flights that normally go through Shannon airport.
Not heard of it being available in Dublin, or for other destinations, or on a single case basis, just flights that have this arranged, the New York flight from LCY being one, they have to refuel in Shannon as the plane is to heavy with a full fuel load, and pre-clearance is done while the passengers have to alight while refuelling.
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