These lines are taken from a Danish ballad called The Ghaist's Warning. A translation appears in an appendix to Walter Scott's poem The Lady of the Lake.
Nelly's couplet isn't quite accurate. It should be:
Twas lang i' the night, and the bairnies grat:
Their mither she under the mools heard that;
An approximate translation:
It was late, and the children wept,
Their mother beneath the earth heard that,
The Ghaist's Warning can be found here with an accompanying glossary.