I had a wonderful tour with Marina, who was incredibly knowledgeable and personable. She was passionate about the history and culture of Dun Laoghaire (one of the first things you learn is its pronunciation!), Dalkey, Dublin, and Ireland, which made the tour that much more enjoyable.
The coastal views, the architecture, and the small-town charms are all highlights and nice contrasts to central Dublin. The train ride from Dalkey to Bray has amazing views (unfortunately, the dirty DART windows obscured them somewhat), and you should spend time walking along the promenade in Bray after the tour ends.
It's a walking tour of about 4~5km, so you do need to be somewhat fit and mobile. There are uneven surfaces, stairs, and narrow passages, so I would not recommend a group tour for anyone with mobility issues. Opt for a private tour or the self-guided tour, so you can move at your own pace. (Unfortunately, regardless of tour type, those with mobility issues won't be able to go up the James Joyce Tower for the amazing views.)
Marina provided a ton of recommendations for things to do, places to visit, and of course, food to eat. You can't do everything, so you have an incentive to return.
As an aside, due to a lost item (which was never found), I retraced the steps of the tour. Without Marina's commentary or the stops (plus the stress of searching the ground), it was less pleasant the second time around, but the views and the architecture were still stunning.
I can't recommend Marina or the tour enough. If you're looking for a day trip just outside of central Dublin, this is a great option.