O'Brien is eulogizing Peter Porter who has passed (see the title, "Leavetaking"), but writing as if Porter were still with him. O'Brien notes the setting: a place where O'Brien have both visited on separate occasions (Chateau Ventenac). "Une pression" is beer (in French). He recalls Porter would have preferred a Minervois (red) wine. O'Brien uses the phrase,

Bad news prefers its poison cold and long...

saying that good wine should not be wasted over bad news (death), but saved until a more appropriate time, after an "acceptable" interval (space of time) of mourning has passed. O'Brien writes that the wine might be enjoyed at midnight—when everyone sleeps—in a spot where a Nazi colonel once sat, waiting...a tidbit of information that Porter would have tucked away to think on later; but gone, "there is no later..."

The author notes that everyone must die, including flute-playing psychopaths. (This may refer to the Hitler Youth, who were heavily involved in music.) But O'Brien refuses to be "reconciled" to the idea that death comes to all: though it may happen—why must it include Porter?

O'Brien describes the scene around him: the boats come into their slips. Last fall's leaves gather around them, and on the deck chairs that speak of "former merriment." Someone starts a fire, and the flames are like poetry. O'Brien stops to ask Porter if O'Brien has included enough details yet, admitting that Porter would have been quicker than he to know when such a mundane gathering would be transformed and become poetry: a piece of art.

The owner of the place (la patronne) enters the scene, critical of the fire. The bartender arrives on his bike to take in the bickering that has erupted, viewing it with quiet humor—it makes him smile. O'Brien compares it all to an artistic event: a poem or play, but a dark one ("black-edged pastoral").

His friend's philosophy was:

The world...exists

Not to be understood

But to demand conviction.

O'Brien agrees, but does that matter? The "dancers" have arrived: party-goers?— accompanied by men looking like George Chakiris (leader of Jets in the film West Side Story).

O'Brien lays out the scene before him: a slice of life—not Shakespearean until he and Porter would study the tableau before them over a beer—when Porter would turn the scene...

...into a poem in the high nine hundreds...

(The Dewey Decimal System assigns 999 to "extraterrestrial," so perhaps the poem would be "out of this world...", especially in that Porter has died...)

O'Brien has not yet learned Porter's lesson. "Work is good...like love and company." However, the "courteous deaths" (those who pass quietly?) do not agree. An obscure reference to Dionysus* and his band of sea women going to war makes me wonder if O'Brien is saying that death uses such women (or Harpies—snatchers—or the Fates?) to collect the dying:

Sent from a place less beautiful than this...

Perhaps these "deaths" cannot understand this world. Maybe O'Brien believes Heaven could not be as glorious as where he now sits.

This place the death or women come from may be in the shade, out of sight, where evening and songs end—for no one is left to sing...perhaps because in the darkness lies the end of all things.

*Addtnl. Source: http://www.theoi.com/Georgikos/Ariadne.html

In general, (as per the introduction) this poem is a tribute to the life of Peter Porter.  As such, I believe there are several "inside jokes" throughout the piece, that are so personal, only those who knew both the author and Peter Porter would likely understand them fully.

Several alcohol references/images are used.  One possible reason is that the poet had a special relationship with this man which included knowledge of fine wines, beers, or simply the love/appreciation of drinking.  Une pression, for example, is a French draught beer and Minervois refers to a specific French wine.  The fact that it is presented as an argument suggests that the two friends often shared friendly banter over appropriate alcoholic beveridges for certain situations.  It also suggests the two drank together.

The poem may present an internal scene of fond remembrance for this friend (who has died) through the external scene of a bar or tavern in France.  The comments about the rugby player, the "patronne" (or boss), the barman, and "cool, pink-pastelled blondes" are painting the scene of what the speaker perhaps remembers from a time he spent with his now dead friend.  In his mind's eye, the speaker presents the scene as if it were a play, unfolding before them, which they were subsequently in the midst of.  The mention that the "players" do not think of themselves "Shakespearean" sounds to me like he's trying to say they were real people, this was a real memory, but it was so artful and beautiful at the time that everyone in the room could have been acting as if in a theatrical performance.

There is a tone of longing, familiarity, and beauty in this poem.  There is also the hint that the memory itself is perhaps colored with the rose-colored goggles that come with drinking, and I doubt this is on accident.  The combination of art and beauty with something so ordinary as drinking in a tavern in France, finally, suggests to me that this friend was down-to-earth and relateable, but well read, intellectual, and possibly philosophical.