"I cannot tell" by William Young Fullerton:

I cannot tell why He Whom angels worship,
Should set His love upon the sons of men,
Or why, as Shepherd, He should seek the wanderers,
To bring them back, they know not how or when.
But this I know, that He was born of Mary
When Bethlehem's manger was His only home,
And that He lived at Nazareth and laboured,
And so the Saviour, Saviour of the world is come.

Wiki has the rest under "Londonderry Air".