Train from Dublin to Cobh after overnight flight from U.S.

If you haven't used www.rome2rio.com you might check out that. It shows a direct bus from Dublin Airport to Cork. While it may have links for then booking, others have advised it is more accurate and reliable to then go directly to the provider website and book there. Enjoy the planning!

Aircoach has hourly departures from Dublin Airport to Cork, takes about 3.5 hours. The bus is comfortable enough, though a train would probably be more comfortable. Weigh that comfort against the hassle of getting from DUB to Heuston.

Check the website, but I believe Aircoach will let you take a later bus, for a small additional prepaid fare, if you should miss the departure time you booked. That's worth choosing in case of flight or immigration delays.

You could take the train all the way to Cobh - maybe spend a night or two there (ie one full day) - and then backtrack by train into Cork where you'll have more rental car options than you're likely to find in the vicinity of Cobh..
A better alternative might be to catch the bus or train to Cork upon arrival in Dublin, get a good night's sleep there, and then collect your car the next morning for the quick drive over to Cobh for a day's exploring. From there head down to Kinsale via the cute little Passage West ferry and be on your way.

I took the bus from the airport to Galway. I took the train from Cork to Dublin. Both were efficient, perfectly comfortable, easy as pie, had wifi, not expensive, and felt great.

Tip: We chose Cork because it was the only town in the region that we could find which had both a car rental agency that was open on a Sunday morning AND a rail station - this surprised me a bit, but in retrospect, most towns are all small, and it's a region/country that still retains a lot of legacy traditions (I assume the tradition was that Sunday mornings are for church, or maybe for sleeping in after enjoying the previous night in a local pub where the craic was ninety). So, check your date/time to ensure all critical services, like the rental car counter, will be open when you need it. On a Sunday morning in southwest Ireland, for us the only choice was in Cork (at the airport).

I don't think you can go wrong with your plan whether you pick the train or the bus (the train would involve one change, but I suspect it would be an easy one, unless you have multiple giant bags, but hopefully you won't make that choice for other good reasons).

Check the drop-off points for train versus bus in Cork (or wherever you're getting off); you may need/want to grab a taxi to get to your accommodations or your ultimate destination for the day (that's what I'd do after moving my butt across the world). Should be pretty straightforward (maybe easier at the train station, I have no idea where the bus drops you) and not prohibitively expensive.

Thanks very much to all of you! I appreciate all the ideas. I'm think I'm leaning towards the train to Cork, spend one or possibly two days there (perhaps with side train trip to Cobh), then pick up a rental car before exploring Kinsale and then on to points west. Thanks muchly.