Hi, domjohnson. I majored in English, and I love questions like yours. Here are the verses and what they mean in modern English.

'Sain' is a verb meaning to bless or consecrate or to make the sign of the cross. related to 'sign.'

'The truth ye'll tell to me, Tamlane,
       A word ye mauna lie;
       Gin eer ye was in haly chapel,
       Or sained in Christentie?'

The truth you'll tell to me, Tamlane, a word.
You must not lie.
Have you ever been in a holy chapel
or blessed by Christianity? (she probably means "Have you ever been   baptized?")
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39I.27       'The truth I'll tell to thee, Janet,
       A word I winna lie;
       A knight me got, and a lady me bore,
       As well as they did thee.

The truth I'll tell to you, Janet, a word.
I will not lie.
A (human) knight fathered me and a (human) lady bore me,
just as a knight and a lady did thee.
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       Randolph, Earl Murray, was my sire,
       Dunbar, Earl March, is thine;
       We loved when we were children small,
       Which yet you well may mind.

Randolph, the Earl of Murray, was my father,
Dunbar, Earl of March, is yours.
We loved (were affectionate) when we were small children,
as you may still recall.
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Whatya think?