Is your question specifically about Ireland's interpretation of "durable partner"? If so, this question will be moved to the relevant forum.
The UK's interpretation of a durable partner is that it is "a relationship akin to marriage", with at least two years of residence together, shared bank accounts/bills/tax statements, etc. The UK does tend to relax this requirement if there are children born to the couple, but even then, a few months residence together is expected.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.