How to pay tax for a part-time remote job in Ireland while employed in the UK as PAYE

Tax resident in the UK and currently employed here, paying through PAYE. Considering taking a short-term part-time remote job for a few days over the course of a couple of months from a company based in Ireland (ROI). I haven't done any kind of freelance/self-employed work up to this point, so what would be the process be of setting up and paying the tax for this particular short term work? It seems like I'd have to fill out a self-assessment form? Is that something I'd have to do before or after the payment as come through? Also, if this is the only job I'd be planning to do outside the PAYE system, which will be at most a couple of months this year. Would I have to be filling out self-assessment forms every year indefinitely due to this? Thanks!

Hi Medallion Pond,
If you take up the opportunity of this self employment, based in Ireland, your self employment income from this job, will be considered foreign self employment income.  If you will be freelance/self employed/sole trader, you will need to register for self assessment as self employed, so that National Insurance can be calculated on your profits, where payable.  
You can register for self assessment as self employed at Set up as self-employed (a 'sole trader'): step by step.
Thank you.

Hi, thanks for your response. Is it still considered foreign income if I conduct this job completely remotely from here in the UK and the payment is sent to my UK bank account directly? The other question I had was whether I'd need to fill the self assessment out before receiving payments or after it's come through? In terms of the NI number, I heard something about getting a Class 2 NI but I'm unsure if this is going to replace my current NI or is just for this self employment part?

Hi Medallion Pond,
If it is paid directly in sterling then no.
If paid in Euros it is foreign income.
You will need to declare it as self employment in your tax return in order to pay the Class 2.
If no foreign tax has been deducted, you do not need to show it as foreign income as it will all fall under self employment turnover.
Thank you.