At the Southern most end of La Rambla, near to Christopher Columbus memorial, you have the Drassanes Metro stop. (Green Line, L3)
Midway along the Barcelona Las Ramblas, just outside the Liceu Theatre, you have the Metro stop Liceu. (Green Line, L3)
At the northern most tip of the Barcelona Ramblas you have metro stop Catalunya which is serviced by the Metro on both the (Green Line, L3) and (Red Line, L1). Catalunya is also the point at which you can catch the Aerobus ( Express bus service to the airport) and the Bus Turistic which will take you around 2 circuits of popular tourist attractions.