littletom-
Some of the issues you raise have been discussed already in the Irish Planning Matters forum (near the top of the main page). It’d be worth your while having a look at some of the threads in there to see if your questions have been answered in detail.
In essence, you can extend your house up to 40 sq.m. (cumulative total) without requiring PP, subject to certain restrictions.
It’s up to you to decide if you want to engage an architect (I might get shot for saying that here). Many extensions are built without an architect. If you do get an architect, they can usually do the planning end of things for you too, if it’s required.
Re costs- the sky’s the limit. More detail would be required before a better answer could be given to that question.
Your best bet might be to ask friends/neighbours how they went about it themselves. Personal recommendations are often the best way of finding an architect, builder, etc.
Hope this helps.