The phone in the US is not $99 - if it is, then you are not comparing like with like. At $99 in the US, this phone is subsidised heavily because it is on a contract and what you are looking at in Ireland will be the unlocked phone. Look on the US Apple store website and look at the 5c, but the unlocked version and you will see that it is a lot more expensive that just $99.
$99 will be a subsidised price, probably AT&T and this means that you are locked into a 2 year contract. Compare like with like and price is near enough the same and you won't save a lot by buying in the US either, by the time sales and local taxes are added and also import duties, you will end up paying the same and also you need to remember that warranty is country specific for the iPhone and NOT international either, so no warranty support for a US phone in Ireland.