Yes it is freely available.

Welcome drink on boarding - sparkling wine is the only alcoholic offering.
Depending on the flight length, there is then a drinks cart run followed by drinks with the meal. On the short west to east night flights, they go straight to the meal service.

If you want more just ask for it. There are warnings about the increased effects of alcohol at altitude, and the crew will refuse to serve anyone who appears intoxicated.

Nick