Well, they did find these at Knowth
http://www.megalithomania.com/show_site.php?site_id=0&image=1052
The inset is an extremely phallic thing. It was discovered at the entrance to one of the passages in an oval setting of quartz stones.
There was a broken, plain version of this found in a similar position at Newgrange.
At a few Irish sites some tablets similar to the Iberian ones have been found, but surprisingly they are more associated with court tombs, which predate passage tombs .... hhhmmm
It has been suggested that the tombs at Carrowkeel could have been painted. There has never been any proof found of this, but the theory was proposed because the limstone there is totally unsuited to carved decoration. Jury's out on that one.
Personally I like the Mr & Mrs Stone outside the one passage at Knowth ... seen here being propostitioned my one Holy McGrail <cough>
http://www.megalithomania.com/show_site.php?site_id=0&image=1054