If you get the bus from Dublin to Drogheda at 8am, it gets in to Drogheda bus station at 9:15, get a plastic breakfast in McD's across the way and the bus to donore/newgrange leaves at 10:15 and takes less than 15 min. Last bus leaves Donore at 17:55 but thats from the village so it will leave Bru na Boinne at around 17:45 and the last tour gets back at 18:15. Last tour might be off during winter though and you may have missed the opening dates for Knowth, I think it closes late Sept/early November for the winter.