Here's a translation of the above words.

I'r helbulus oer yw'r biliau
Sydd yn dyfod yn y Gwyliau,
Gwrando bregeth mewn un pennill,
Byth na waria fwy na'th ennill

To the troubled cold (i.e. chilling) are the bills
Which come during the holidays;
Listening to a sermon in one verse;
Spending more than you earn.
(I don't think I did this verse very well. s,ww)

Oer yw'r eira ar Eryi,
Er fod gwrthban gwlanen arni,
Oer yw'r bobol na ofalan',
Gwrdd a'u gilydd ar Nos Galan,

Cold is the snow on Snowdon,
Although it wears a woolen blanket,
Cold are the people who don't take care
To meet each other on New Year's Eve.

Remember, there was/is a tradition of taking a tune and singing a lot of different verses to it, for fun. Similar to a lot of campfire songs we used to sing at camp.

sian