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