If you sign it and return it then it would be a binding contract, until then they could retract it. Seems very remiss on their part though. On further reflection though, I suspect that AIB, like most Irish Banks these days, have a clause for fixed rate mortgages which states that the rate applicable will be the rate on sale on the date of drawdown - i.e. the 5.39%, not 4.39%. This clause allows for sudden large increases in funding costs for the banks to be passed on to the customer if necessary. Before you say "sharp-practice" it makes infinite economic sense. If in any other business the cost of supplying a product or service increased dramatically, you would look to pass on that cost.
Keep copies of everything, and be prepared to complain if the bank increase the rate.
You have two options.. the first is to sign the form, and hope no-one notices, and then complain if they notice in the future,
or
ask for clarification on the rate before signing.. if you do this the rate may go up, and you're stuck,.. or they may confirm the incorrect rate, in which case you sign and your complaints are much stronger in the future.