Meaning of "let the old and weary sleep" in Byron's "The Bride of Abydos"

Question

I am unsure about the meaning of a passage from Lord Byron's The Bride of Abydos (Canto I, stanza 3).

1. How are we to understand the sentence between dashes: let the old and weary sleep below?
2. What cannot Selim (one of the characters) do when he says I could not?
3. What is the subject of Were irksome below?

“Father! – for fear that thou shouldst chide
My sister, or her sable guide –
Know – for the fault, if fault there be, (55)
Was mine – then fall thy frowns on me!
So lovelily the morning shone,
That – let the old and weary sleep –
I could not; and to view alone
The fairest scenes of land and deep, (60)
With none to listen and reply
To thoughts with which my heart beat high
Were irksome – for whate’er my mood,
In sooth I love not solitude;
I on Zuleika’s slumber broke, (65)
And as thou knowest that for me
Soon turns the Haram’s grating key,
Before the guardian slaves awoke
We to the cypress groves had flown,
And made earth, main, and heaven our own!

Full poem here

Answer 1

I understand it to mean something like:

The morning was shining in such a lovely way that I was unable to sleep, unlike those who are old and tired. But I don't like being alone, so the idea of viewing the fairest scenes of land and deep with no one to listen and reply to my thoughts irked me. So I woke up Zuleika and we went to the cypress groves before the slaves woke up.

How are we to understand the sentence between dashes: let the old and weary sleep?
What cannot Selim (one of the characters) do when he says I could not?

Here, he's saying it's fine to let old and weary people sleep in the morning, but he couldn't sleep.

What is the subject of Were irksome below?

The subject of "irksome" is the action of viewing the fairest scenes of land and deep with no-one to listen and reply to his thoughts.

Answer 2

1. “Let” here means “permit, allow” (OED sense 12). We have to supply some elided words in order to make this a complete sentence, for example, “[although we] let the old and weary sleep, I could not [sleep].”

   Selim now admits that, unable to sleep in the early morning (“let the old and weary sleep”), he broke in upon Zuleika’s sleep in the harem.

   _{William H. Marshall (1962). The Structure of Byron’s Major Poems, p. 42. Philadelphia: University of Pennsylvania Press.}

   The dashes and elisions help convey Selim’s nervousness in confronting his father.

2. See 1.

3. In “to view alone … were irksome”, “were” is subjunctive, “expressing a hypothetical condition” (OED sense 20). This sense of the word is now archaic: we say “would have been” instead. There is no subject as such: to put the sentence into conventional order we would have to supply a dummy subject, for example, “it were irksome to view alone”, where “it” is the dummy or “expletive” subject.

   What Selim means is that if he had viewed “the fairest scenes” etc. on his own (“alone”), it would have been irksome, because there would have been “none to listen and reply”, and so he woke Zuleika instead.