Living in UK (NI), working in ROI

I have registered with HMRC as resident in the UK now and my self-employed income will continue to be in the ROI.

My annual tax return will still be submitted to Revenue but I will also need to do the same to HMRC. I understand the HMRC financial year is April to April and that I will get credit for tax paid to Revenue.

How does this work in practice - when will I need to submit the first tax return for April 2019-20 and how will it work thereafter?

My ROI financial year is concurrent with the calendar year - how will this be allowed for for HMRC ?

Thanks in advance.