What does "It's I'll be here" mean?

Question

I found a song where I don't understand the meaning of it is:

Or when the valley's hushed and white with snow,

It's I'll be here in sunshine or in shadow,

and

The summer's gone, and all the roses falling,

It's you, it's you must go and I must bide.

What does this it's mean?

I suppose it could be similar to this is, or it means. Am I correct?

Answer 1

There is an omitted relative pronoun in each of these lines.

It's you, it's you who must go and I who must bide.
It's I who will be here in sunshine or in shadow...

Today omitting the relative pronoun is common only when it stands for the object of the verb in the relative clause:

It's you who I want to see.

But in older English omitting a subject relative was common, and that use survives in dialect and much colloquial use:

You know the people who own the house on the corner?

The author employs this obsolete syntax to lend his song a traditional flavor, just as he uses the obsolete pronoun ye, and the obsolete 'subjunctive' tread in the fourth verse, and sets the song to an old Irish air.

Answer 2

It's is simply used here to describe a situation, not much different from it's a sunny day. However, what follows is not standard.

You could read it as follows (I put the verses in order):

It's you, it's you must go and I must bide.
becomes
It is now like this: it is you that has to go and I will have to stay

and the second verse:

But come ye back when summer's in the meadow,
Or when the valley's hushed and white with snow,
It's I'll be here in sunshine or in shadow,
becomes
But if you come back, when it's summer,
Or when the valley is quiet and white with snow,
It will be like this: I will be here (with you) in good or bad times,

So the song describes the current situation, and a promise of a future (possible?) situation, that if the beloved returns, the situation will be different.