While it would really help to have additional details, there are a few things you can read to try to answer some of your questions.  You have, obviously, seen the Irish Rail web site, but this link lists several options for tickets:

http://www.irishrail.ie/faresandtickets

This is a NON-Irish Rail web site that offers some insite, as well:

http://www.railusers.ie/passenger_info/ticketing.php

All Irish Rail ticketing info is only valid 28 days ahead.  For 'Guesstimate' purposes, you can use choose a corresponding day of the week that falls within the 28 day period to determine an APPROXIMATE rate for individual ticket prices.  You can also consider the Tourist Passes, which include Rail travel throughout Northern Ireland AND the Republic ...

Online Prices can run as cheap as 10 Euro per ticket and can be purchased as late as the day before your departure -- but the earlier you buy, the better the choices for travel times.

Irish Rail travel is pleasant, but unremarkable.  Trains generally run on time and nearly all of the trains have been replaced with new, Green, state-of-the-art vehicles within the past two-to-three years.

Bob

shifter,

While we can help answer train and bus questions, asking how much a flight costs is - unfortunately - beyond us. If we ran the airlines we could tell you (if we knew where in the US you were flying from and to). But there are many airlines and many fares, which change constantly. I'm afraid you will have to contact the airline of choice for that info.  

My article about How Much Does a Trip to Ireland Cost? might be useful.

Bob & Tony have provided you with lots of other info to help. 

Michele

I wasn't looking at flight costs, but the trains.  My trip isn't until Oct and the rail sites only go out 28 days in advance