Italian immigration say as long as you have a stamp from entering France and you haven’t exceeded your 90 days they are happy.
Could get a wee bit tricky for UK residents who are Irish Passport holders and their spouses/immediate family members who enjoy exemption from the 90 days stay limit. The new EU Entry/Exit System should manage the process, but, in the interim, there have been reports of Border Officials stamping/not stamping passports; so anyone who travels with an Irish Passport holding spouse and then wishes/needs to travel to EU using their own 90 days Allowance will need to ensure that stamps are applied for the joint travel, and will be wise to keep emails of ferry bookings etc so that problems around overstay when travelling solo can be corrected without drama. The EU Border Guard Technical Manual instructs Staff to offer travellers the chance to have the record corrected at the Border by showing proof of the earlier joint travel
Steve