In just three lines, the poem captures what a first drink can do. The speaker takes a gin, and almost at once finds themselves "in the ice." That could mean the cold of the glass and the drink itself, but it also suggests a feeling of being frozen, suspended, or shut inside something clear and chilly. The title, "Lush life," adds an ironic edge: "lush" can mean rich and luxurious, but it is also slang for a heavy drinker.
The last line is the strangest and most striking. "Here is night in a sack" turns darkness into something you could pick up and carry, or something you might be stuffed into. I read it as a feeling of being closed in, perhaps by loneliness or the numbing pull of drinking. The speaker sounds neither joyful nor in agony, just flat and observant. The mood is cold, compressed, and quietly uneasy, with a lot left unsaid in very few words.