Drimnagh is the main one. The Druids' Judgement Seat was a great loss - even though it does provide much amusement today :-) A second tomb near to Ballybrack and a wedgetomb near to the coast just north of Dun Laoghaire spring to mind as two lost monuments. There's a standing stone inside the porch of the Norman church next to St Patrick's Cathedral, which I entirely forgot about!