As a Dub some advice:
* Bring arm warmers and a jacket for the bike. It may be twenty something degrees Celsius, it may be 8 and raining.
* Don't go on the lash before the race no matter how many pubs "that serve the best Guinness" you have to go to :)
* By and large a friendly welcoming bunch in Dublin however there is an element of street crime, be careful with your belongings as you would in a US city
* The Phoenix park (where the run is) has lots of false flats

Enjoy

If the race is early in the morning, make sure you set up a cab/transportation to the event in the morning. Usually public transportation isn't running that early. Saw a lot of people walking through Zurich, a few miles from the start, this summer at 4:30am going to the start of the Ironman.

+1. Try and stay close to transition. Public transport in Dublin isn't anywhere near as good as bigger cities. For example there is no underground train system

Its a split transition with some distance between transitions.

Oh right, the swim is in Dun Laoghaire. I don't know what the organisers are putting on, but here's a few tips from my experiences (born there, sister lived south of Dun Laoghaire for a while, now lives near Phoenix Park, been all over the city by foot, bike, taxi, bus and train - that said, not as much over the last 18 months):

• The train to Dun Laoghaire from Tara Street starts at 7am normally. My guess is they will make this earlier as I believe the local government were really into having an Ironman event at the city.
• It's a 3km walk from Tara Street to the edge of Phoenix Park.
• Taxis are pretty common although like any city sometimes there will be a squeeze. (Have walked all the way from St Stephen's green to Blackrock (7km) without seeing a single free taxi until I was almost home). Book them ahead of time, and schedule them to arrive 30 minutes before you actually need to go. (Not used Uber there yet though)
• The standard buses to Dun Laoghaire from the city centre mostly use bus lanes to get in and out of the city, but expected travel times (e.g. from Google Maps) may vary depending on general congestion.
• Renting a car is probably a good idea, but if you come from a US city with a grid, you'll need a GPS navigation device or a generous data plan on your phone or you are likely to get lost. If you drive and stay near Phoenix Park then consider taking the M50 to Dun Laoghaire rather than going through the city. It's longer but faster if there's no construction/work going on.

Given it's the inaugural event it would be great to get one of the RDs on here to advise on travel. I'd say your best bet is:

• Consider staying to the east of Tara Street at one of the hotels (Spencer/Clarion/Jury Inn) as you can walk into the city centre (1km-ish), can walk to the train, etc. At the very worst you can cycle to Phoenix park (3km) to drop your bike off and then walk back. This is a better option for those that are bringing families who will want to be in and around the city.
• Consider staying near the park (or to the west of it) and book a rental car as a backup if the race doesn't have good transport options.

In terms of getting to your hotel from the airport there are loads of taxis. The AirCoach is also a good option which is cheap, reliable and regular (http://www.aircoach.ie/faqs). They allow bicycles (fully-assembled) depending on space, but guessing everyone will have bike boxes.

If that was the case the original decent route would have gone ahead

Wow, if they don't put on any additional transport options, it's likely to be a bit of a shitshow if you ask me

And this is exactly why i'm asking...as I have this feeling. I have no clue how the Transitions will be set up, but will need to get run to Phoenix park, and bike to Dun for the end of swim. Wondering if I could just bike over and drop off in the a.m....but that'll make for an early go of it.

Was worried about "space" in cabs for bike cases and such as well. The Tri All isn't small...

Thanks everyone for the info so far!! It's much appreciated.

I did Rhode Island 70.3 (2012, I think) and it had a split transition. I vowed never to do a split transition event again because of the additional stress (finding multiple race sites, figuring out where to take bike vs run bags if different, finding out where to pick up swimsuit stuff after the race, getting transport to race start, etc).

Sorry to be a downer, I'm sure if you read the race program and book cabs ahead of time you'll be fine. Biking to Dun to drop the bike off is probably a good idea. Find out if they will have a rain cover for overnight storage. If you park it in the a.m. you have 24 hours for rain to come and leave you with a squeaky bike!