Quite a heroic escapade. I felt like doing the same there on my first visit last week.

Knowth was the first place I've visited where I got really angry. The hour-long guided tour is not nearly enough – I begged two hours out of them. The 2 sealed passages – aaarrrgggghhh. The iron bridge over the kerbstones into the cutting and the complete blocking off of the eastern passage to facilitate this. The ugly and inaccessible constructions over the satellite tombs/observatories.

This from the top of the mound:
http://www.themodernantiquarian.com/post/48076