To pay PRSI you need to be a "chargeable" person, where you need to do a Form 11 for your taxes.
Sometimes described as someone who makes over 5k a year in non PAYE income - which isn't the complete picture.
has more details - according to that document, Deposit interest > 5000 but taxed at source is an exception (to becoming chargeable), N26 isn't taxing at source - though it seems it would be a good thing if they could.
I'm Form 11 but my understanding is it's relatively easy to pay on MyAccount/Form 12 - but Revenue are currently sometimes handing it incorrectly and charging too much tax. If it was me I'd try to hold off filling in that form for a few more months and hope they fix it - they are admitting it's an error it seems - which is good as admitting errors wouldn't be their strongest point.