Exhausted of irony, the poem folds back into a prosaic description of the sweepers' morning. The thousands of sweeps who had risen "upon clouds" to "sport in the wind" now rise to the brutality of their daily life: "And so Tom awoke and we rose in the dark / And got with our bags & our brushes to work." Tom's dreamwork, however, has done its job, and in the final couplet's summation of his innocent--and thus mistaken--attempt to extract intelligibility from the vision, he draws a meager and tragic consolation: "Tho' the morning was cold, Tom was happy & warm, / So if all do their duty, they need not fear harm."
Indeed Tom's "duty"--filth, disfigurement, and probable death--is the only recourse open to him. But the banality of the last line won't bear much looking into; certainly it isn't meant ironically, unless it's the pure intratextual irony of Tom's and the speaker's now convergent acceptance of oppressed misfortune. In a poem of social protest, intratextual irony is insufficient (at least for all but lit-crit-hardened graduate students). Whatever poignancy and pity the reader may feel is deflected by the implicit anger that Blake communicates toward an economic and religious system that weaves us all into its warp.
More likely, we're meant to take the appeal to duty straight-up. On October 7, 1803, Blake writes from London to his sometime-patron and friend William Haley:
O that I could live as others do in a regular succession of Employment this wish I fear is not to be accomplished to me.... [N]othing is necessary to me but to do my Duty & to rejoice in the exceeding joy that is always poured out on my Spirit. (Erdman, p. 737)The word "duty" recurs frequently in Blake's letters, less often in his poetry. (If you've reached this page via another route, you might want to have a look at the online Blake Concordance.) The word rarely bears the disparaging connotations that one might expect, given Blake's artistic and religious Dissenting opinions.
In "The Chimney Sweeper," a heartbreaking poem if ever Blake wrote one, we're carried into a world of unredeemed, and unredeemable, innocence, whose tragedy is heightened by a failure to carry its outrage into the active world of experience. A London without the warmth of the hearth would be unthinkable, and, without the sweeps, there would have been many Great Fires. Blake and Catherine were childless, but, by all accounts, the poet got on well with children. And yet, he, too, probably had his chimney swept.