Would be nice to see some more airframes on the ramp even if only for pre-clearance. Though if airline(s) were to land for pre-clearance and pick up passengers it would expand the options for getting to the US.
But what about getting back from the US? The benefit of an SNN stop westbound is clear (whether being able to offer pre-clearance can actually translate into extra passengers or revenue for the airline is another question) but what about eastbound? Unless there's a significant passenger flow to/from Shannon, an airline choosing to stop there is inconveniencing all of its would-be non-stop passengers from the US to [somewhere in Europe]. So for example BA if their LCY-JFK service actually gets off the ground will only stop in SNN westbound.
Now if SNN could persuade Jet Airways to move their BRU hub operation to SNN, *that* would provide some volume...