I should probably have said yield per passenger rather than yield.
The catchments of Cork and Shannon are fairly similar in population. Cork is maybe slightly larger, but there's not a huge amount in it.
What's different in Cork is the number of people who live very close to the airport. Cork's catchment includes some densely populated areas around the city and some sparsely populated areas in West Cork and South Kerry. Shannon's is more uniformly spread.
A 20 minute travel time includes the whole of the city, Carrigaline, Glanmire, Ballincollig etc. and also the rural area down towards Kinsale.
On the other hand, in 20 minutes from Shannin, you're only just hitting the outer suburbs of Limerick.
While the overall catchment can indicate demand for flights in general, the lower the time spent traveling to the airport, the more likely someone is to be happy with an early morning flight.