If one has dual nationality, is there any substantial and/or legal reason for requiring to maintain both of them as valid ?

A friend who has a vaid Irish PP believes she must keep her R.o.P PP valid for travel to R.o.P.

I would have thought if a Filipino national, who can prove that by other paperwork, even though she travels and arrives in RoP on an Irish PP, is entitled to enter the county without any formality and stay for as long as she wishes, even permanently.

Legal views on this would be appreciated

John