I'm so glad you came across it, I'm assuming not many would know about it but if you search Amazing Grace it should pop up I do come across it scrolling .
Yes, I wondered that as well, could be since that's the first of it's kind in this form, Was it Songs of Praise idea at the time!
While this is Mairead and Eabha's only for Susan she has done it twice interestingly and the exact same song for Postcards!
They also did You Raise Me Up in 2015 alongside. I'm glad you posted the Danny Boy link because its hard to find until two people posted on YouTube. If I found another source of Danny Boy I would shared the link below...maybe someone can help I don't if someone can download it and share it on youtube or link.
You Raise Me Up (featuring Lynn 2015)
https://youtu.be/j3-_tN2roro?si=dvB9W5Wzv9NkzUsL