Cars can be rented at the Liberia Airport through Expedia, however, be aware that there an many hidden insurance fees in Costa Rica that you will have to pay when you arrive. Be sure to check with your rental car company about these so you are aware before hand.
There are buses available from Liberia to Tamarindo, and from there you can take a taxi to Playa Junquillal. The 1505 Bus leaves hourly from Liberia and costs approximately $1 USD
Taxis are available for hire from Liberia Airport, however, an amount should be per-arranged. Fares will vary, but be should less than a private shuttle, and more than a bus. (Approx $50)