Usually Illegal (ie 2 x UK PP) BUT if you have ROI ancestry you can legally ( I think) have an ROI (EU) passport and a UK one.It might be handy having two passports you would still have to exit schengen after 90 days , but could go straight back in with second passport ?
I do know of some who chose to get an ROI passport following the exit vote in 2016.
With a ROI passport unlimited time in EU !!!!!
Not so seriously....
Trev get across that border now !
Seamus maybe Ok as well