Why is the approach different in Germany vs. Ireland?

Before the ePrivacy Directive and GDPR, Germany had an Anti-Competition law (Section 7 Para. 2 UWG 2010) in place that requires opt-in consent to send marketing emails. This law doesn't explicitly state you need double opt-in consent, but the burden of proof is on the sender, and local courts have often found that single opt-in is not sufficient to prove consent, as a third-party could have entered the email address of somebody else. Therefore to prove consent the company need to have sent the double opt-in confirmation email to the email address and only when the email address owner has clicked the link in the email to confirm their consent is it considered valid proof of consent.