I don’t quite know what ‘luminously-peopled air’ means – perhaps that the air was full of the sounds of luminous people.   Taken out of context, I can see it as the air being full of spirits, but that may be moving too far from Larkin’s poem:

Luminously-peopled air ascends;

And past the poppies bluish neutral distance

Ends the land suddenly beyond a beach

Of shapes and shingle.

Here is unfenced existence:

Facing the sun, untalkative, out of reach.

From ‘Here’ in the ‘Whitsun Weddings’, which can be found … here.  See also Joshua Weiner’s article about the poem.

Source: Seamus Heaney, Finders Keepers: Selected prose 1971-2001 (London: Faber and Faber, 2003), p. 149