I am developing a website in Ireland and hosting it on American web servers.

Am I subject to Irish or American laws regarding the content?

It depends on what you're doing. If you're selling things, it's very easy to insert a clause into a contract of sale that reads something like: "This contract is governed by the laws of Ireland"

If it's libel actions you're worried about, it's more complicated. Basically, libel is publishing untruths that damage somebody's reputation. Under Irish and European law, it seems (it's not fully clear yet) that you can be sued in Ireland for all damage done to their reputation worldwide or in any other country for the damage done to their reputation in that country. The European courts concentrate on the place where the damage was done (which can be anywhere) and the place where the event that caused it occurred (i.e. you (in Ireland) uploading it to the server). The physical location of the webserver is unlikely to be important. Unfortunately, I know nothing about US law, which might decide to claim you too!

Either way, if you want real legal advice, see a solicitor.