Monopods arent really tripod replacements, but as the original question suggested tripods may not be allowed then they beat hand holding! If you go to Newgrange and catch one of the guides in a really good mood they might turn a blind eye to one or two sneaky pics, theres no way in hell they would let you bring a tripod in but if you have a quick release head on a monopod you could easily rest the camera on it and take a shot in a few seconds.

If they let you bring in your own torch and you have an accomplice you can take handheld shots with a good chance of getting a few sharp ones, all the photos of carvings in this gallery ( http://cianmcliam.smugmug.com/gallery/347643/1 ) that dont have **new** in front of them are from Loughcrew and Newgrange and were all taken handheld at around 1/5 to 1/8 sec at F3.5-4.5 with the camera set to ISO 1000 with the only support being a handy stone to lean on. If I had a monopod that day I would have a lot more, and a sharp one of the tri-spiral in Newgrange! All you need is someone to wave a torch from below or the side of the carvings.