There is a machine at each DART station where you key in your destination and it will tell you the price of the trip. Put your coins in and you get your ticket. However I have to say that these machines are not user friendly as not everyone knows the destination or name of each stop. There usually is also a human being that you can talk to who will also sell you a ticket.
I'm not sure if any of the hop on/off tours pass your hotel. The buses usually park around St.Stephens Green and go from there mainly around the city centre areas.
The DART trains are very roomy and bright with plenty of space for a childs buggy. At peak times they are packed with people going/coming from work. Be warned however that the "ticker" that tells you the next stop on the DART is usually broken and often gives the wrong information. However each train station is clearly marked with signage. There are also clear route maps in each carriage so you can anticipate each stop.