Happy birthday to your mum!
If she's into gardening, you could take her to some of the bigger garden centres. My mother loves a trip to The Orchard in Celbridge, and we occasionally take a trip out there, and have lunch in their restaurant. I'm sure there are other garden centres with restaurants if Celbridge doesn't suit.
I haven't been on the Dublin tourist buses, so I can't comment on those, but I have been on the Ghost Bus tour and thoroughly enjoyed it. If your mother enjoys history, and doesn't have a problem with hopping on and off buses then it might be worth considering.
Some Dublin hotels such as the Westbury, the Gresham and the Clarence do Afternoon Tea, which would be a nice treat.