What is this tune/lyrics?

Have just watched Tylluan Wen movie on You Tube and heard a song sung at point 2m:17sec. Can’t for the life of me recall where or when but remember it from way back in my youth. Anybody shed any light on this catchy tune’s title? Diolch.

Anyone who went to a Welsh school, English or Welsh language, would have sing this at some point in their childhood!
I remember it well, but had to look the title up! “Cwcw lon” apparently.
“holia ci ci a holia cwcw” was the chorus I knew, but they seem to be singing “holia ci ci a holia nos da” if I heard right.
There’s a lot of variations on it, mind, along with variations on actions to go with it (and one girl there seems to ensure that “nos da” is the phrase used in that chorus!)


You’ve put me out of my misery - diolch yn fawr