My confirmation email doesn't state that there is a time limit. Have you received the confirmation email stating 21 days?
I'm assuming you booked under the Bookcrossing event. If you wish I can contact the event organiser at the Maldron and clarify the situation. I would need your full name and dates you have booked. PM me the details if you'd like me to clarify.
I need to get them to resend the email, I can't find it at all. I'll get back to you when I have it. Thanks!