I totally understand the confusion! USIT is an Irish organisation that provides discounted travels services to students, hence why Bus Eireann mention it as the place to purchase the card - but that is for Irish students. You can get yours from a French website, or isic.org like you were planning to, the card will be equally valid.
Apart from students fares, have you thought about the Open Road Travel Pass
? Depending on how much you intend to use Bus Eireann buses around the island, it could result in a better value!
You can also have a look at our article on bus travel
, you may find some useful info there.
Do you already know which places you want to visit? If you let us know, we'll definitely give you some pointers for all the unmissable things to see and do!
Have a lovely rest of the day!
PS: vous pouvez tout à fait poster en français si vous préférez. :)