Actually I'm from Monastery too (like alot of the previous posters it seems). I lived in the old main Monastery estate growing up (about 28 years) and have since bought a home with my fiance in one of the newer Monastery Estates adjacent to it (for the past 3 years). It is the only area I wanted to live in. We were actually considering selling, but have recently changed our minds, as we don't want to leave. If you get anywhere along the monastery road you will not be disappointed, it is a very safe and well cared for area and I would have no concerns for my fiance's safety if she was walking home at night, unlike some other areas. Indeed some of the estates have competed in the Tidy estates competition and have consistantly ranked very highly at the top level, even competing against areas in Blackrock, Dundrum etc.
There are other good areas such as New Road, St Brigids. Try and stick to what I call the Old part of Clondalkin if your budget will stretch to it (they are pretty expensive), but personally I would never consider buying in Riversdale which is located in the Village as it can be a little dodgy at times - this is down to a minority of troublemakers as usual (but this is just my own view - I have some friends who enjoy living there and have had no probs at all). Perhaps you might get an apartment in either the Shalimar or Mount Talbot complexes. These will be a good investment imo.
Woodford / Dutch Village would also be an option as they are close by too, and many go for quite reasonable prices second hand, but they wouldn't be as nice as the other areas mentioned - but not too bad either (the prices reflect this)
I don't really know the other side of Clondalkin that well, so I cannot comment on it, other than to say that when I went to school that's where the all the tough kids came from! (make up your own mind on that one). That said, you could live in one of those areas and never have a problem. BTW I wouldn't go beyond the Mill towards Neilstown. If it's past the Lough and Quay, forget it is my advice. (The area has improved in places, but is still very 'hit and miss').