score:5
Since both Ireland and Germany are in the same customs union, any customs check is unlikly.
Also this not a matter of endangered species but of shells commonly found on a sea shore, may be considered a natural process.
There are countries or areas which don't like certain natural products being 'stolen' (Sardinia and other vacation islands)
So the first place is to check Irish Customs
But asking in Ireland would be wise.
This German site Sand und Muscheln aus Ferien mitnehmen however also advices asking locally and
The original German Customs site, however, uses the phrase giant clams
bis zu drei Exemplare von Riesenmuscheln
so this may not be the same and then only for personal use and not as a gift.
Upvote:3
As you are no collector of shells it is very likely you will not recognize protected species, especially as shells can come to the beach far from where they live.
If there is a wildlife ranger or other specialist available to you in Ireland, ask.
If they are a protected species it does not matter you found them already dead, as there is no visible difference between your shells and shells fished, killed and cleaned for collectors.
If in doubt, go through the red channel, something to declare, when you arrive in Germany. It will take a bit more time, you may even only find a phone to connect to a customs officer on some locations, but you will not be charged a fine if you are not allowed to take things through. And officers have some leeway allowing 'honest finds'.