Check with your airline. Different carriers have very different baggage limits, so if you haven't bought your ticket, that might be part of the selection process. For the international flights I've taken, the bike counted as part of regular checked luggage, and there was no extra charge. For domestic flights here in the US, there is usually a $50-80 charge to check the bike as baggage. Either way, you'll have to box the bike.
I looked into UPS'ing my bike to Australia, but it was very expensive. DOn't know what the charges would be from Ireland. If you decide to go that route, I bet you can find a bike shop that will receive your bike for you, especially if you pay them to reassemble it.
You can pack all your panniers together into a big duffel bag, and either carry it with you on your bike or send it to yourself at the end of your ride. In the USA, you can send a package "general delivery" to a post office in another city, and they hold it for you. Check with the USPS web site for how long they will hold it.
Have a great tour!