Lines 49-60 Summary

Get out the microscope, because we’re going through this poem line-by-line.

'O look, look in the mirror,
O look in your distress:
Life remains a blessing
Although you cannot bless.

Our clock-speaker is turning up the intensity here. The repetition of look really makes it feel like he's desperate for us to hear and understand.
He wants us to take a good hard look in the mirror, knowing we might not like what we see. This echoes the clock-speaker's request from the 10th stanza that we "stare, stare in the basin." This guy really wants us to see ourselves. We have already been shown (and even told in line 24) that Time conquers all. Now, we must accept the fact that this means Time will conquer us (and love as well). We may end up feeling, well, distressed.
It can be a hard pill to swallow: this idea that life is a blessing, but, no matter how we might plead or pray, we are powerless to "bless" ourselves or anyone else with the power to escape Time's ultimate triumph over earthly life. That rhyme between distress and bless really reinforces the feelings of futility by tying a feel-good word to a negative word.

'O stand, stand at the window
As the tears scald and start;
You shall love your crooked neighbour
With your crooked heart.'

In the previous stanza, the clock-speaker wanted us to look at ourselves in the mirror. Now, he wants us to stand at the window and take a look outside.
The mirror lets us see a reflection of ourselves. The window lets us see the world outside ourselves. Anyplace we look, Time wins—hence those scalding tears.
This stanza marks the last words from our clock-speaker (evidenced by closing quotation marks at the end of line 56) and it feels like he's trying to end on a positive note… sort of.
The clock-speaker ends by telling us to embrace Love, even if it is imperfect (crooked), frail, and temporary in comparison to Time. The clock-speaker wants us to go ahead and love each other as best we can with our crooked, mortal, doomed hearts.
Okay, maybe "positive note" was too strong, but you get the gist.

It was late, late in the evening,
The lovers they were gone;
The clocks had ceased their chiming,
And the deep river ran on.

Our poet-speaker, the guy from stanzas 1 and 2, returns to get the last word. Here again, he is reporting his observations of the scene.
In the poem's first stanza it was evening, perhaps sunset. Remember how that image of "harvest wheat" put the golden sunset color in out minds? Now it's dark. Really dark. The repetition of late makes sure we get the picture.
The lovers are nowhere to be found. They are gone in a literal sense, but also in a figurative sense, too. Consider this: darkness often represents death. Darkness has overtaken the scene. The lovers are gone from the scene, but they are also, figuratively, dead.
Not only have our lovers kicked off, the clocks are silent, too. They have "ceased their chiming."
Time is no longer being recorded.
The tracking of Time, the counting of minutes and hours, is a manmade pursuit. Even after no one is left to keep track of it, Time will run on and on, kind of like… yup, you guessed it, that deep, dark, eternal river.
And even when the poem ends, the river runs on.
The poem begins "down by the brimming river," and ends with, "the deep river ran on." It's no accident that, even as the poem comes to an end (as the clock-speaker told us everything must) it ends with the deep river of Time running on and on.
Well played, W.H., well played.