In case anyone is wondering why Kyrie eleison is sometimes Englished as “Lord have mercy upon us”, blame Thomas Cranmer, Archbishop of Canterbury under Henry VII and Edward VI. He was one of the greatest prose writers in the history of the English language, but (as he, himself, knew), he was a vile poet, and actually believed that “Lord have mercy upon us” could be sung to the same music as Kyrie eleison because they both have seven syllables. (Yes they do. If you think they don’t, you’re not pronouncing Kyrie eleison correctly.) Suffice it to say that I don’t know a single musical setting in both Greek and English that has the syllables underlying the music in the same way in both languages.
Link to the article: What does “Kyrie eleison” mean? (A Straight Dope Staff Report by SDStaff Songbird, Straight Dope Science Advisory Board)
For those wondering: Kyrie is pronounced key - re - ee - ay. For example.
Uh, no. Key-ree-ay ay-lay-ee-zohn. The first word has three, the second four.
How does that help with the second line in the chant (replace “Kyrie” with “Christe”)? I suppose you could drop the “up” in “upon”, but then you’ve set many of the syllables in the second half of the line to different notes in each line.
I don’t know offhand what he might have intended. As I say, he was a Michaelangelo of prose, but when it came to verse and music, he was utterly cloth-eared.
I’m not so sure about that. The Koine Greek Wikipedia page says that the ei diphthong started being lost as early as the fifth century BCE. And this invocation can’t predate the first century CE. The actual vowel chart suggests it actually has an “ee” sound.
So the original pronunciation would be /ky ri e e li son/ or /ky ri e e li zon/. Or, in English respelling, kü-ree-eh eh-lee-son or -zon, with ü being that rounded ee sound.
Of course, the songs were written in Latin, not Greek, and a Latin transliteration probably used Latin vowels, not Greek ones. But the Latin of the day, Classical Latin, appears to have pronounced ei as a diphthong. And, worse, two vowels in a row across words were often elided. That would mean 5 syllables: kee-rih-eh-ley-zon (/kiː rɪ ɛ lej zɔn/).
That’s not to say that songs weren’t written with seven syllables. I’ve definitely been told that “Latin doesn’t have diphthongs.” But it does seem the phrase has either 5 or 6 syllables as originally pronounced, if said Wikipedia articles are accurate.
I am unaware of any liturgical music using the phrase that puts kyrie eleison into six or five syllables. I’m not an expert, however, so perhaps someone can point to such an example from prior to Thomas Cranmer’s existence on Earth. 
I’m traveling, and will be for over a week, so I’m in no condition to respond to this. However, I can definitely say that 20th–21st-century Anglican music uses seven syllables.
Bach’s B Minor Mass (1720s?) uses seven syllables. Padilla’s Missa (1710s?) seem to use six, by eliding between the two words. They both pronounce the two diphthongs with two syllables each. Hard to tell for sure because of the melismas, but that’s how I was taught to sing 'em.
The same first seven (guitar) notes in “Stairway to Heaven,” I might add. 
(Certain live versions, anyway…the studio version ascends on the first four-note phrase.)
Aw, I was hoping for a Lord Of The Flies clip.
I always like this Mr Mister song: Mr Mister - Kyrie Eleison - YouTube