It's a funny lyric. An article about the song suggests that it may have been meant to be sung by a lover, rather than a mother:

Who is Singing to Danny?

Suggestions have ranged from an Irish chieftain bidding his son farewell to a female friend. Given that Weatherly had stated in an addendum to his 1918 publication of 'Danny Boy' that when a man sings the song, the words 'Danny Boy' should be substituted with 'Eily Dear', it may be that the song is only meant to be sung by a woman - perhaps to avoid embarrassment among males singing the song in those days. Many suggest that it has to be sung by a lover, taking the line 'And I will sleep in peace until you come to me' to indicate possessiveness uncharacteristic in a parent or mentor.

Maybe the singer is saying she expects a great fulfillment from Danny's arrival in the afterlife. But surely the notion that souls sleep in the afterlife until a final fulfillment is not Catholic, but a minority Protestant view. As it happens, the same lyricist wrote the sentimental song "The Holy City", which is often suspected of Masonic associations.

The tritone B-F is part of the chord G-B-D-F, which contains all the notes of that echo "sleep in peace"; there's an even greater dissonance on "come", in which the chord is G-A-B-C-D-E!

So, that means no 7th chords, et cet.....

I bet you flatted a tire, too.

Actually, it seems that a fourth was augmented. Just sayin'.

I think the reference in the Alto line is the augmented fourth drop from the B natural to the F natural in the first measure.

Basically, this King's Singers arrangement is something of a cross between jazz and barbershop, so the weird notes & harmonies aren't unexpected. It's a world in which almost anything goes.

Thanks for the correction!