I would avoid dealing with them by phone at all, simply because if you ring them they could pressurise you into agreeing to make regular payments larger than you can actually afford, plus you don’t want any phone contact from them at all. It’s best to do it by post, & is less intimidating. Also, by doing it all by post you have an exact record of everything, which you should keep.
Ideally you would be dealing with the company/financial institution to whom you owe the money, but it may be the case that the debt has been sold on to them, or it could be the case that the letters have been coming from the credit control department of whoever you owe the money to, but you didn’t give much detail in your post, so it’s hard to say.
Anyway I suggest that you send a registered letter to the collection agency & acknowledge that you owe the money to the original company/financial institution. Inform them that you are presently unable to work due to ill health & that your sole income is a social welfare disability payment & are struggling to make ends meet given your current situation.
Inform them that you have made an appointment to see a MABS Money Advisor but there is a waiting list for an appointment. Tell them that, considering the circumstances that you will make the effort to pay €5 per week until such a time that MABS can fully assess your circumstances & tell them that you would be grateful if they would suspend any interest charges or further action until such a time that MABS contact them on your behalf.
Make sure you do pay them €5 per week, & keep a copy of all correspondence sent & received. Meanwhile make sure you make an appointment with your local MABS office. You may want to ask the disability allowance section of DSFA to send to a letter to you stating that you are in receipt of payment from them, a copy of which you could send on to the debt collection company. (or if you still have the letter that DSFA sent you telling you your claim has been approved & what your weekly rate is, you could send them a copy of that with your letter) It would also help if you could get MABS to send you a brief letter stating that you have applied for an appointment to see them, which you could send to them also.