I think the line is actually "And stay in bed ON TIL Sunday". Many Irish use ON when talking about staying in bed. For example, "I'll be lying on" means you plan to sleep late (or as in the US, we say "sleeping in"). Also there is a bit of a gap between the ON and the Til, when Dolores sings this line. BTW, I visited her grave when we were in Ireland this past May (2018).
I think the line is actually "And stay in bed ON TIL Sunday". Many Irish use ON when talking about staying in bed. For example, "I'll be lying on" means you plan to sleep late (or as in the US, we say "sleeping in"). Also there is a bit of a gap between the ON and the Til, when Dolores sings this line. BTW, I visited her grave when we were in Ireland this past May (2018).