"He runs for a day."
Translation:Ritheann sé ar feadh lae.
the expression is "ar feadh lae" (feadh = duration) and that would be sort of "for the duration of ..." which is a genitive expression in Irish, but better translated into English with a non-genitive expression.
(a) During, throughout.
Ar ~ an lae, during the day, all day.
Ar ~ bliana, for the space of a year.
Ar ~ tamaill, for a while.
Ar ~ an ama, na haimsire, sin, during all that time.
Ar ~ i bhfad, for a long time.
Ar ~ na faide, all along.
If you scroll down into the appendix, where it shows the use of ceann, then you see:
go ceann » Fan go ~ seachtaine, wait for a week.
go ceann » Ní bheidh sé agam go ~ míosa, I won’t have it for a month yet.
go ceann » Go ~ tamaill eile, for another while.