Now that's just downright crazy, and there's no sense in scaring someone for no reason. I took a 2 month solo tour in Ireland a few years back, and had absolutely no issues getting around by bike, even in major cities. Roundabouts make sense and are EASY to negotiate, if you don't approach them in the sort of defeatist, paralyzed fear described in this post. Use your brain, and you'll quickly get into the rhythm of local traffic, it should become second nature during the first day of cycling.

I'd recommend getting a mirror (helmet mount is my preference). Those giant tour busses are eerily quiet, and were the only vehicle I had trouble hearing until they were actually passing me.

And traveling alone in Ireland is easy and a pleasure. Stay at a hostel, and you'll make instant friends. Visit a small pub, and if you're even halfway friendly people will be chatting you up. You'll likely meet cyclists from other countries too.