Think I've solved the mystery. Its the radon gas;
http://glasnost.itcarlow.ie/~feeleyjm/dolmens/carlow-dolmens.pdf
Also looks like you'll have no problem getting in if you go up there twirling a couple of rods ;)
This bloke seems to have gotten in fairly recently;
http://www.megalithicmonumentsofireland.com/COUNTIES/CARLOW/Ballynoe_PortalTomb.html
He's been fairly busy. Links to your site as well.