"Oft, in the Stilly Night (Scotch Air)" is a rather somber and melancholy poem in which the speaker remembers friends he has lost and reflects upon his own loneliness without those friends.
In the second stanza of the poem, Moore says that he has seen his friends "fall, / Like leaves in wintry weather." In this simile, Moore compares his friends to the leaves on a tree. Just as those leaves fall in winter, so, too, his friends have fallen, or died, in their old age. In this simile, the "wintry weather" represents old age. The speaker's friends have died in their old age, and now he is left all alone. The simile comparing the deaths of his friends with the falling of leaves suggests that these deaths were natural and inevitable.
The speaker also says that he remembers these friends "so linked together." This image suggests that his friends and he had once formed a close-knit group. Thus he cannot remember one of the friends without at once being reminded of the others. The fact that these friends were "so linked together" when they were all alive emphasizes the sadness of their deaths. Each friend fell, or died, separately, just as leaves fall one by one from a tree. The implication here is that these deaths were lonely. This idea of loneliness is also emphasized in the subsequent lines, when the speaker says that, his friends now dead, he feels "like one / Who treads alone." The speaker is the last one left of all the friends, but ironically, and tragically, the fact that he has been left alone is a kind of death in itself.