If you want to be heinous and drink two cans of beer on your flight you better not fly a 'full service' airline anymore as getting that second can is like pulling teeth.
You get far better service on the Irish LCC you mention as all you need to do is wait for the frequent cabin crew trips down the aisle (remember when you got those on full service airlines??) and ask for a can of beer. It will cost a couple of quid, but then that's what beer costs in a pub, or a cross channel ferry, or a train, so no idea why it should be any different on a plane.
You do indeed get far better service on a LCC if you quite reasonably like to drink a few beers on your flight.
And depending on the registry of the aircraft, it can be illegal to drink your own booze on board. It's illegal to be 'drunk' on an aircraft, which of course depends on the definition of 'drunk'. In some places it means being over the drink drive limit.