Cross border working

bordergirl21

I've looked this up quite a bit online and have the basics, but would like some info of an actual real life experience
I've been offered a lucrative post across the border with HSE In Donegal, currently work in NI civil service and the salary across the border will more than make up the extra commute, and will allow me work reduced hours.
A few questions I have going through my head:
Has anyone experience of owing tax to HMRC, as I've read that the tax on employment income is less in the south and there can be a shortfall? I am assuming that when claiming tax relief on my hmrc tax return for the universal social charge payments that this would offset this?
The married person tax allowance down south, can this be claimed if my partner works in the North?
As my role will be involve significant travel as per the role, I will be claiming mileage allowance, the HSE rates are above and beyond civil service ni rates, will this be treated as taxable income for hmrc once I go over the 45p allowable hmrc mileage rates, and is this submitted on the tax return?
I have also read that as my residence in the North and resident country of our children, we will still be eligible for family allowance, and also I am on PIP for a heart problem i have had since childhood, will this be allowed to continue?
I am looking constructive information please, no negativity or Brexit discussions -I have lived within a 5 mile radius of the border my entire life and I already know the Brexit implications