Kerry has 10 yrs +++ of having HAhn. Kerry is a German favourite.... Outlived German services to SNN and NOC over the years. Kerry is a better fit for the Grrman market.....
KIR-HHN is at the lowest frequency it has been for a decade, summer and winter (2 weekly winter, 4 weekly summer). If you speak to HHN based crew, KIR in winter is is not where you see anything close to 90% loads.
HHN has a pretty wide catchment area. Not only Germany (the wider Frankfurt area, and most of NRW to the north), but also Belgium, Luxembourg and parts of France. You will even see plenty Swiss cars in the HHN car parking lots.
Back in the early 1990s before HHN was open to passenger traffic, you had a Saturday/Sunday Aer Lingus operated FRA-KIR (via DUB in at least one direction).
Berlin has a very different area catchment area and no history of links to the west of Ireland. It might generate a little bit of outbound holiday traffic from Kerry but I cannot see that accounting for much.