Translation:I will be in work at eight o'clock.
In American English, we do not say "in work" we say "at work". I reported this nuance but do not know if it matters to anyone.
To be in work = to have gainful employment. I think it should be "I'll be at work at 8 o'clock"
Shouldn't it be "I will be at work at eight o'clock"? I would use erbyn for by, not am.
I'm not sure what you mean, the two sentences I can see are "Bydda i yn y gwaith am wyth o'r gloch." and "I will be in work for eight o'cloc". Am in this instance is used to mean for. I would also say "I will be at work by eight o'clock" as "Bydda i yn y gwaith erbyn wyth o'r gloch".