Re: AIB Student MasterCard
While as you say it would be desirable to pay off the entire balance of EUR 400 if you don't have it then you don't have it.
The terms and conditions of that card allow you to get away with making a so-called "minimum payment" well in time (to avoid the automatic direct debit), the exact figure appears at the end of your last statement, maybe EUR 20.
Yes, you will be charged interest for the "loan" of EUR 380 but at least you will have made the minimum payment and kept within the terms and conditions of the card