American living in Ireland with a Green Card. If I apply for long-term residence, what are the benefits?

I know that I can apply for long-term residence when the two years are up for my Irish green card. I want to know what are the benefits of doing that. Can I move to another EU country and not need a new work permit? What are the benefits of long-term residence to me, other than entitling me to live in Ireland for a prolonged period? What do I need to do to be able to move freely within the EU?