Us "Non-North Americans" dont know what you mean by B&N. I'm gonna assume its a book shop of some sort. Anyway...

You can't borrow any book, but you can buy any book.

It's a matter of what you need and your financial situation. If you cant afford it — borrow it (borrowing from the librabry is completely free in Ireland with no late charges).

If you need multiple books of a single category e.g., for studies — borrow it. Is it a specific book? You might only be able to buy it, and want to reread it or use as a refernece here-and-there — buy it.

The library doesn't have everything, but the interent does. But maybe this B&N place does. But I'm not sure what that is.