Orchard Garden Centre in Celbridge is good and I agree Johnstown is excellent. Newlands in Clondalkin is always worth a visit too.
For shrubs, you could try Choisya Aztec Pearl - evergreen, about 4 ft high and masses of very fragrant cream blossoms for weeks in late Spring. Mine is beside a dwarf 4.5 ft lilac. A Sarah Bernhardt peony is easy to grow; it dies down in winter but comes up in early Summer with huge fragrant pink flowers.
A hebe is a good border shrub - fairly low, rounded and dark evergreen. For contrast, a New Zealand flax in soft green/salmon stripe or dark pink - lights up the garden all year. I have a scarlet Oriental poppy set against a mint green shed - not boring!
If your garden gets good sun, the Just Joey rose (big orange scented blooms on bronze-green foliage) would be beautiful alongside a Margaret Merrill cream fragrant rose.
If you are doing hedging, a combination of green laurel and perhaps Photinia Red Robin is very pleasing to the eye.
Good luck with your planting.