The meaning of "Bhí dóchas aige go dtiocfadh Sara lá arna mhárach" is more like "He was hopeful that/He expected that Sara would come the next day". To express a bit more certainty you could say "Bhí muinín aige go dtiocfadh...", which is closer to the English "He was confident that...".

I meant to write "dtiocfadh". Ooops. :)

I got curious and used Google to translate the sentence aND the result was : He was confident that Sara dthiocfadh next day.