Well, the command not to charge interest on loans in the Old testament was only within your own people: e.g. a Jew shouldn't charge interest from a fellow Jew, but he could charge interest from non-Jews as much as he liked.

Now, the Christians view themselves as the "new chosen people", so they couldn't charge interest from each other, so the banking system had to be performed by Jews, who could - in clean conscience and following their religious beliefs - loan/charge interest from non-Jews(Christians).

In short, the whole "irony" is lost once you actually study the specific commandments and the historical context of the described situation.