To get to Ilha do Guajiru, you need to go on the CE-085 highway. Unfortunately, Google road maps are incorrect in north east Ceara area, so you can't count on navigation apps. Read the instructions here and in Brazil Transportation section, download the map below, follow the signs, and you will make it to Ilha do Guajiru.
Go on CE-085, for 190km north west of Fortaleza at the direction of Itarema. You pass Pecem, Taiba, Paracuru, Lagoinha and Flecheiras till you reach Itarema. In Itarema, follow the signs to the beach (Praia), at the roundabout in front the lagoon, turn right, to reach the main spot. CE-085 is easy to reach from Cumbuco, Taiba, Paracuru, Flexeiras and Lagoinha, but can be tricky to reach from Fortaleza airport.
By 4x4. You can drive at the beach while Downwind, from Cumbuco, Taiba, Paracuru, Lagoinha, Flexeiras and Icaraizinho. Driving conditions at the beach are dynamic. Transportation options depend on tide, seasonal lagoons and more. Local driver is a good choice. Kitesurfing down wind into Ilha do Guajiru lagoon, is tricky, and can be done only in high tide conditions, so please read Kitesurf Icaraizinho section, and better do that kitesurfing trip, with guided escort.
Bus company Redencao (Tel: +55.85 3256.2728 / 3494.4744) runs few daily busses a to Jericoacoara, via Itarema, from several stops in Fortaleza. From Itarema take a taxi for 20R$.
Bus company FretCar (Tel: +55.85 3402.2244) runs daily busses to Jericoacoara from Fortaleza, last one departure at 18.30. From Itarema take a taxi for 20R$.
Mind that from the airport, you might need a ride to the buss station. For more details, read Transportation section. The trip takes around 4h30, and costs 20R$.
By taxi, that can be arranged in any pousada, or right outside the aieport. Price depend on the taxi company and the type of the car (4x4 is more expensive)(4, good estimation for price per regular car, is: