I would decipher that along the lines of,

Mochyn du sydd wedi marw;
Mam a Dad yn crio'n arw.

I'm sure I've heard the full verse somewhere but it isn't in the book I have to hand. Indeed, it may be one of the verses that have been added in 'the folk process' so it might not be written down in any of the formal collections.

I'm interested in your Nain being born in Patagonia. The inference is that she moved from there and you're resident of elsewhere. I'd be curious to know a bit more ...

sian