You have this service from the Airport to Plaça Catalunya, with a frequency of about 10 minutes (approximate travel time: 35 minutes.).
From there you have two options:
Metro: you must take the metro (Line 3, direction Trinitat) to the Diagonal stop, where you must change (Line 5, direction Vall de Hebrón) to the stop Sant Pau | Two of Maig.
Bus: you must take bus 47 to the Industria – Avenida Gaudí stop (9 stops) just 3 minutes walk from the apartments.