Well technically both are on the Liffey flood plain but since the Leixlip Dam was built way back when flooding hasn't been an issue downstream of it (Ringsend et all were flooded by storm surges, not the liffey).
I've seen the occiasional rat in Chapelizod but this is to be expected at any riverside location, you're fooling yourself if you think otherwise. Pretend they're big voles/mice and they don't seem so bad. I wouldn't touch a ground floor apt in either location because of this, but like I said this is to be expected and I'd be loath to touch a ground floor apt by any riverside location. It's a clean stretch of water but rivers = rats.