It is top round of pork leg, bone in or bone out, that is brined or pickled for several days.
It may then be smoked over smouldering peat or juniper.
In Ireland, you can buy the ham already cooked and ready to reheat for an hour or so, then serve, or in already cooked packaged slices.
Consumers in Ireland beware, though: “The term Irish sliced ham is used to describe ham from outside Ireland that has been sliced in Ireland; sliced Irish ham is Irish ham that has been sliced.” 
Needs cooking, usually by baking.
Soak whole hams overnight.
Small pieces of ham, you can get away without doing this
Can be quite salty, so you may wish to simmer it first before baking it or finishing it off in the oven.
Serve in thin slices.
 Cullen, Paul. Not quite what it says on the tin. Dublin: Irish Times. 24 May 2010.