I've bought lots of things from Maternity Revolution at Brendale (Jillian mentioned the full address). They're not cheap per se, but about the same as any other clothing shop, e.g. Rockmans, etc. They've got a huge range and stock lots of different brands.
There is one on the Southside called Joy's Maternity on Loganlea Road, Waterford. They have quite a range and it's resonably priced (average price for pants is $40-$60) and they have a small second hand section.
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