If you are wearing a mask in the city centre, you certainly won't stand out. There are lots in shops, and plenty of people wear them on the streets as well. (I am in relatively frequently due to work).
Phoenix Park, outside the zoo, would be a nice place for a long walk, see the deer etc. Also, Collins Barracks national museum is nearby, which might be interesting to DS. And being slightly outside city centre, wouldn't be too manic.
If you do want to shop/cinema/food court etc, there is also Liffey Valley Shopping centre, just before the M50. Although I haven't been there in a number of years so I don't know what shops are there now. But would mean avoiding city centre.
I agree with others about traffic close to Christmas and in rush hour. I'd also suggest pre-booking your parking at the airport. If there is none in the car park right next to the terminals, the Red car park is actually fine to use as the buses to it are frequent and it is very easy to get to from the M50 and to get back on the road afterwards, and pretty well lit as well.
City centre coming up to Christmas, especially if the pubs have not been closed again, will probably be busy as there was not that much gathering last year, so lots of people are talking about doing a lot this year. So there will be a lot of meals/pub outings going on, shoppers, and general movement of workers and tourists. It has definitely not recovered to the levels it would normally be at this time of year, but it is already busier than it was this time last year. So bear that in mind. But there are lots of great places to visit and shop and eat, just do your research in advance to know where you want to go.