The Chimney Sweeper by William Blake : can you please explain each line in the poem and what are the ironies and imageries and symbols in the poem

In your question about Blake's "The Chimney Sweeper" you ask for a bit more than we can answer in one answer (suggested length 90 words), but I'll give some information that will help you make sense of the poem.

First, I'm assuming you're studying the poem from Blake's Songs of Innocence rather than the poem from Songs of Experience of the same name, so I'll answer your question about that poem.

In short, the first stanza gives the young boy's history (the speaker's history).  The details are an accurate depiction of how boys were sold into virtual slavery to become chimney sweepers in Blake's time.  Some sweeps were orphans and some were sold by parents who couldn't afford to raise them.  The "occupation" usually led to an early death from inhaling soot.

The speaker then tells the story of another sweep, Tom Dacre, who cries when his hair is cut, which was the usual practice to keep soot from getting into a sweep's hair. 

The speaker tells Tom not to worry because cutting his hair will keep the soot out--in other words, he gives him a rationale.  But the point is really that he is taking the point of view of the exploiting adults in the situation and the point of view of society.  The speaker is essentially telling Tom that "It's for your own good." 

The remainder of the poem presents society's view, and the church's point of view, suggests Blake, that Tom should not worry about the abuses he suffers now, because he will be rescued when he goes to heaven.  The speaker and Tom are naive and they buy into the line of thought that justifies their suffering of abuses.

In the poem, then, two sweeps adjust to their situation by looking forward to their future rewards.  The reader is left to infer that they are being naive by buying into a line of thought that justifies their being abused.  There is no irony on the part of the speaker because he isn't aware that he's being naive.

Concerning symbols and imagery, Tom's hair is "curled like a lambs back":  innocent, pure.  This simile, image and symbol establishes the innocence of young boys made to be sweeps, and points forward to their naivete.

The black coffins represent the chimneys and the black bodies of soot-covered sweeps.   

By the way, in the second poem of the same name, the chimney sweeper, though also a child, is more aware of the issues involved and is not so naive.  Though still a child, he understands how sweeps are being abused.