As written by David Sottimano in a comment to my post about International SEO, first consider this:
"Avoid automatic redirection based on the user’s perceived language. These redirections could prevent users (and search engines) from viewing all the versions of your site." - support.google.com source
Ask your client this: is it absolutely necessary to deliver the Irish user exclusively to the Irish site? Should the Irish user be able to see other sites?
If the answers are yes and no respectively, you can get away with IP sniffing & server side redirects as long as you do not force redirection on American IPs (Note: Gbot still crawls from the US, but if they change, this won't work). The downsides; usability issues, and the fact that American IPs can see anything they want.
You could also use client side redirection (javascript) with even American IPs with no real consequence (Gbot shouldn't* execute the JS). This is a bit more flimsy, and won't work at all with JS disabled users. Maybe use http://www.wipmania.com/en/api/ for example, and maybe test to see what pisses people off less?
Said that... I would not use geo redirections, but present a box hover or bar (as it does Google when you visit, for instance, Google.it from Finland) where asking if the want to visit the .eu site or prefer to visit the US one (and vice versa). That way you are sure your site will be crawled with not problems (at least not this problem).
For the duplicated content issue, check out what I write about the rel="alternate" hreflang in my post here on Moz.

