gongey,
depends on where you are going to..
There is an interprative centre at Newgrange now but it's on the southside of the river. I think (not sure) you need to visit the centre in order to gain access to Newgrange and Knowth.
The centre can also be accessed from the Northside of the river. This may suit you better as I assume if you are cycling that you will be coming down the N2 through Ashbourne and Slane. I can give you detailed directions if you
want.
Advantage of travelling through Slane is that you can go visit Dowth. Dowth is the 'ugly sister' of the three tombs but is actually quite a lovely and peaceful place to visit especially if you can get there early in the morning.
As regards whether or not Newgrange is worth visiting, that's a hard question and largely depends on your own tastes. If you are in the area, I'd suggest taking in the Hill Of Slane and perhaps Old Mellifont Abbey if only for the cycle through some quiet country lanes. Again, if you need directions, I'll be only too happy to help.