The fastest way to get from the Cancun airport to Puerto Juarez from where the ferry to Isla Mujeres leaves is to take a kind of a van or taxi but it costs about 40$.
The second option is a shared van which is about 25$.
One can also take the Ado Bus to Cancun which is 5$ per person and then use a taxi to Puerto Juarez.
The Ultra Mar ferry from Puerto Juarez to Isla Mujeres leaves every 30 minutes and costs 11$ per person both ways.