Fairly easy to find, I'm sure you know dun laoghaire a bit?
If you're coming into the town from the coast road, dart station on the left, take a right at the crossroads, up towards the shopping centre. At the top of the road, you go right and immediately left, and it's just in on the left.
Called Mike's Bikes and it's on Patrick St. I'll post a google map if you want?