You can go two main different ways. You could go through
Limerick city, and then on to Patrickswell, Rathkeale,
Newcastlewest, Abbeyfeale, Castleisland, and then Killarney.
Another way is to head to Killimer in County Clare and take the
ferry across the Shannon Estuary to Tarbert in county Kerry. You
can then go to Castleisland, and then Killarney. There is not much
different in terms of distance, whichever way you go.