I have checked what references I have to hand and can find no mention of what these statues may have been. They are not mentioned in McCready's 'Dublin Street Names' (1892) in which he devotes a section to statues. My guess, and it is only a guess, is that they may have been statues that were once inside the building and for one reason or another put out onto the lawns.
What puzzles me is the word 'crumbling'. They must have been made of a very porous material - such as limestone - to 'crumble'. Even monuments in graveyards rarely 'crumble' - they might weather, but not 'crumble'. I have seen freestone 14th-century effigies that have laid in church yards for centuries, but they have not 'crumbled'.
I will, by the way, be spending a couple of days in the Gilbert Library early in September and will see if I can shed any more light on the subject. The answer must lie somewhere . . .