There is no doubt that the new "normal" will see many more people working from home, and for some people that may mean their actual home country as well.
If you are going to work in Portugal "full time" then your employer may obtain a PAYE Exclusion Order authorising him not to deduct PAYE or USC from your payments. You would obviously be liable to Portuguese taxation.
I know some employers are wary of employees working from home in certain countries, (such as France) because they are concerned that they might get caught for a much higher level of Employer's PRSI if the appropriate structure is not established.
As Purple suggests, you should seek advice from an accountant. (Your employer should also seek advice.)
Jim Stafford