Traveling abroad

I am a US Citizen, and about 4 years ago I was denied entry into the UK. I had just graduated from college and before settling down to a job and an apartment in the US, I had planned to travel through England and Scotland to see first hand the country I had studied in college (History degree). The reason for the entry denial was they thought I was going to work there, as I had planned to stay for 5 months, even though this was not the case.

I have not traveled abroad since this incident, and have recently had an invitation from a friend in Ireland to visit. My passport still has the entry denial stamp in it, and this is the only entry in the passport.

Is there anything I should do to prevent this from happening again? I would like to visit my friend in Ireland for about 7 days.

Should I renew my US passport and get a new one even though it won't expire for 5 years? I'm afraid the denied entry stamp from the UK will throw up red flags for the Irish immigrations person. Plus my picture looks nothing like I do now, so I'm afraid that would make them look closer.

Should I apply for a visitor's visa even though I'm not required to have one to visit Ireland?

Any advice would be much appreciated.

[Law200]

getting the visa will assure you that no problem will exist

but the circumstances have changed so you would not likely be denied now if you have a job and residence