Hi, There are no toilets on the buses (Bus Eireann or Ulsterbus). The bus stations have them, even the little stations.
I have found all parts of the buses to be equally comfortable, and Irish roads are very good. However, if your are travelling on a very rural route in the west of Ireland, you may want to sit closer to the front. Some of the bog roads have a lot of dips and bumps - but it doesn't sound like you'll be on one of those.
On most buses, people load/unload their own luggage. Some drivers do it themselves. However, don't hesitate to ask a driver to assist you. On the express routes, the luggage compartment is low to the ground, and it's fairly easy to push your bags in. If you have trouble unloading them, ask the driver (sometimes they slide back). On other buses, the compartment is higher up - but I managed (57 yr female, traveling alone).
I've never seen anyone tip the drivers - usually they're gone before you can thank them!