Smock Alley have a really helpful “Find Us” area on their website.
When I visited the venue last week, their events coordinator said that occasionally Google Maps sends people to the rear of the building which is a fire exit. If you follow the directions on Smock Alley’s website, you should get to the front of the building which looks like this:
There is a bus stop right outside (I was standing at it when I took the photo) which looks straight out onto the Liffey.