TvF is correct. In the original publication of Ulysses, the first page is a great big letter S, with a tiny comma, then the word 'plump' - next page - Buck Mulligan came... so, the word stately is an adverb, referring to how he came down the stairhead - it could have said, "In a stately manner, plump Buck..." Plump is the following adjective, describing Buck physically. "Plump Buck Mulligan is the description. Who walks stately. The confusing thing is, without the comma, the sentence would also work. But in that case, since stately can be either an adverb or adjective, "Stately plump Buck Mulligan came down..." would describe Mulligan and being both stately and plump in appearance. In the actual sentence, he's plump in appearance and stately in the way he walks down the stairhead. Which makes him a more comical figure than the other way of reading it.

TvF is correct. In the original publication of Ulysses, the first page is a great big letter S, with a tiny comma, then the word 'plump' - next page - Buck Mulligan came... so, the word stately is an adverb, referring to how he came down the stairhead - it could have said, "In a stately manner, plump Buck..." Plump is the following adjective, describing Buck physically. "Plump Buck Mulligan" is the description. Whose walk is stately. The confusing thing is, without the comma, the sentence would also work. But in that case, since stately can be either an adverb or adjective, "Stately plump Buck Mulligan came down..." would describe Mulligan and being both stately and plump in appearance. In the actual sentence, he's plump in appearance and stately in the way he walks down the stairhead. Which makes him a more comical figure than the other way of reading it.