Your 48 numbers will work there.
You need membership but your membership allowances will not cover any services while abroad.
You'll need cash credit to cover outgoing and incoming calls and outgoing texts unless you buy EU roaming add-ons.
If you have EU roaming calls add-ons then incoming calls will be free and calls between your mobiles will use the bundled mins.
Ditto for calls to Irish or French mobiles/landlines.
But obviously you get limited minutes on the add-ons.
48 don't support data roaming abroad so WhatsApp will only work on WiFi.
If you are going to use the mobiles a lot then I think you might be better off getting two local sims for the trip.
Eg see here:
In that case you will obviously have to use the new French number for the trip, your 48 sim/number will not be active (unless you have dual sim phones) and anybody calling from Ireland will have to pay for an international mobile call.
Hope this helps.
Ps If you have iPhones then make sure to read this
Sorry about the typos. I hate posting from a phone or tablet... :-(
Edit: typos fixed.