I was wondering about using the train.
I just checked the routing on Google Maps and while it may be a shorter time compared to Viking's shuttle by the Google Maps' estimation, I walk slower than the Google estimation. So for us, it is 50-50 -- and Viking shuttle gets us closer to where we want to be.
Regardless, now that I see what the shuttle schedule is, I am glad that I booked I booked our tour at the Little Dublin Museum for 11 am, because with the tendering, I don't see us arriving much before 10am, even if we are on the first shuttle at 9am.
Here's the link to Irish Rail for timetables and tickets: https://www.irishrail.ie/en-ie/