The plowman homeward plods his weary way, And leaves the world to darkness and to me. (3-4)
The speaker isolates himself from the very first stanza. He watches the last people still out and about—the "plowman" who was working in the fields—make their way home, and enjoys the feeling of solitude in the churchyard. He also seems to feel a sense of possession of the churchyard. Now that he's alone, he says that it has been left "to him." What's up with that, do you think?
Beneath those rugged elms, that yew-tree's shade, Where heaves the turf in many a mould'ring heap, Each in his narrow cell for ever laid, (13-15)
The speaker makes a point of telling us that each individual dead villager is completely and totally alone in his grave. He goes so far as to call each grave a "cell," as though the dead villagers are prisoners in isolated prison cells or monks in solitary monastery cells. Death is isolating; everyone dies alone—we've all heard the clichés. But remember: they weren't yet clichés at the time Gray was writing!
Far from the madding crowd's ignoble strife, Their sober wishes never learn'd to stray; (73-74)
The villagers described in the poem lived their whole lives in relative isolation. They lived far away from the noisy, "madding" crowds of cities like London. They spent their entire lives among a relatively small number of people. Does this mean that they were more prepared for the isolation of the grave than people who live in cities who are used to noise and crowds?
Along the cool sequester'd vale of life They kept the noiseless tenor of their way. (75-76)
Again, the villagers are described as "sequester'd," or protected from the outside world. Their town is insulated from the noise and bustle of modern life.