You're missing the main point of mnemonics, which is simply to help recall. Anki is designed to reinforce memories, but any additional context and connections help this recall.

The best way to learn is really to combine both: make once the effort to build a mnemonic for things that you don't remember easily or which you expect might end up confusing - and then use anki to reinforce this memory.

Our brain works with connections, and connections take effort to form, so all you do to form a single personal mnemonic by definition helps you build these connections, with the mnemonic then helping you later to again find this chain of neural connections that means 'flag of ireland'.