This is what I did. I had all my USMLE's done before intern year and could have matched, but I wanted to stay for the intern year for a variety of reasons (pay was good at the time, registration with the irish medical council) and it was a very good year. In my year (2007), 3 of us did this. There were a couple other Americans who also stayed, but they stayed in Ireland more longer term and came back to the US later.
You have 6 weeks off for vacation. 3 weeks in the first 6 months and 3 weeks in the 2nd 6 months. I did interviews in December by scheduling 2 of my vacation weeks then. I spent a 2nd two weeks packing / moving because I didn't get the 2 weeks off in June that I needed to move back. So instead, I used the 10 sick days I was allotted. This is not really fair to the clinical team I was on at the time, but they were very understanding of my situation and the HSE did not need to be notified of this.
Intern year is NOTHING like an intern year in the US or Canada. Intern year means you do 6 months surgery (3 month blocks in different surgical specialties) and 6 months medicine. So I did Ortho, Gen Surg, Medicine for the Elderly, and General Medicine/Rheumatology. You're on call q7 - q9 and although it's a pain in the ass, you learn a ton on call.
When you go back, you have a very steep learning curve because the systems are obviously different, you have to re-learn drug names, you have to learn how labs a resulted (different acronyms are used) and there may be some procedural things that you're not used to, but the Irish work ethic and the way you learn in Ireland is very conducive to producing stellar US residents.