In a shallow bowl, whisk together beer, flour, and salt.
Rinse fish, and pat dry. Cut into 10 equal pieces.
In a large saucepan, heat 1 inch oil to 360 degrees F (168 degrees C). Using a fork, coat fish pieces in batter. Slide coated fish into hot oil in batches; adjust heat to maintain oil temperature. Fry until golden, about 2 minutes. Lift out with a slotted spoon, and drain briefly on paper towels; keep warm. Repeat to fry remaining fish.
Stack 2 tortillas. Place a piece of fish and 1/2 cup cabbage in the center of the tortillas. Garnish with mayonnaise, lime wedges and salsa