Luxembourg has a lowish tax regime, so for example a married couple earning less than €32,000 may not have to pay any tax (depends on number of children/other variables). Assuming that this is your only rental property there, then you earn €10,200 per year. I'm not sure about this, but the revenue may be referring to the fact that you will file a return of €0 in Luxembourg and then proceed to pay tax on the income in Ireland as normal. It's quite a complicated tax system though, so it's probably best if you speak to a local accountant.
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