It's bad, but maybe not all that bad. You may have to wait a while in the visitor's centre but there's a reasonable exposition there to divert you for a while. Knowth and Newgrange are a good bit apart and you have to come back to the visitor's centre from either one to get to the other – a pain in the arse, so I'd try to get there early, assuming you want to do both on the same day.

The tours to each are an hour's duration. Last time I was at Knowth I asked to be allowed to stay for longer. Knowth has more to see so 2 hours there, if they allow you like they did me, would be my recommedation. I avoided their guided tour, but had to join it at the stage where they enter the mound to get a look into the east passage.

Maybe the biggest tip of all would be not to expect too much – they are great places, but, in my experience, can be a bit of a disappointment due to the rushed nature of the tours.

Hope you have a great time!

Andy