As far as I know, there's no reason why you shouldn't keep your .ie domain name if you decide to change from a ltd company to sole trader. Firstly, no-one's going to check; secondly, sole traders are entitled to a .ie too. I have one for my sole trader business.
My husband registered his business as a ltd company 18 months ago. The main idea was to maximise our tax-free company pensions. Unfortunately, with the recession, we can't afford to pay ourselves salaries (I'm co-director), let alone have anything left over for pensions. Bad timing, I guess, but hopefully things will improve.
You should be able to get some decent advice from a thoughtful accountant on the financial pros and cons of de-registering your company. If yours isn't providing this, ask for recommendations and find one who will. Or see if your local Enterprise Board can come up with the name of a business advisor.