While Yeats doesn't actually mention the name Robert Gregory anywhere in this poem, everybody knows that there's only one Irish airman that matters in his poetry, and that's Robert Gregory. He's the speaker of "An Irish Airman Foresees His Death," and he features in several other poems in The Wild Swans at Coole, the volume of poems in which Yeats first published this poem. In The Wild Swans at Coole, the poem immediately prior to "An Irish Airman" is called "In Memory of Major Robert Gregory," so that once one gets to "An Irish Airman," Gregory is definitely in the air, so to speak. Yeats actually wrote two other poems about Gregory, one called "Reprisals" and the other "Shepherd and the Goatherd." You can read "Reprisals" here and "Shepherd and Goatherd" here.
The Wild Swans at Coole as a whole is in many ways all about Major Robert Gregory. Even in poems that aren't ostensibly "about" him, his loss is felt. Thus, in the volume's first poem, "The Wild Swans at Coole," Yeats ruminates (or ponders) old-age, death, and loss—themes that are most certainly informed by Robert Gregory's untimely death at age 37.