Hi Patti,
If you can get a ride to the train station, it makes more sense to go to Killarney. if you have to take the bus, the only way is to go to Cork and then continue to Dublin. I usually use the
Rome2Rio website to calculate public transport itineraries, it might come in handy for you.
I hope you'll make the most of your day and evening in Dublin. If you arrive by mid-afternoon in the city, you might be able to check out attractions (Trinity College, St Stephen's Greens) or simply walk along the Liffey and soak in the atmosphere. You could also catch a traditional music session at night at
one of these pubs.
Talk soon,
Antoine