The best advice is to be organised early!
Accommodation is booked up quickly and by January you should know your budget and arrange your accommodation.
If you wish to rent an apartment for the week in Cannes two of the biggest rental agencies are http://www.homelidays.com/ and http://www.holidaylettings.co.uk/ or you can organise it through the Cannes Lion team here.
Most people arrive at the Festival via the Nice Côte d’Azur International Airport, located less than 30km from the city of Cannes. Aer Lingus and Ryanair both fly direct from Dublin airport to Nice Côte d’Azur which is the nearest airport for Cannes.