Hi Bondfitz
Having worked for a small firm in a similar trade, we used the following packages:
Big Red Book for accounts (this worked when the staff compliment was under 3 - once the company got bigger, it began to show some shortfalls)
Collsoft for payroll (excellent value and very simple to use, never had any problems)
The biggest problem you will encounter is valuing your work and expertise. I would advise that you contact the RIAI and enquire as to whether there is a small firm, or another member prepared to mentor you in this matter. It would also be worth your while investing in some manner or form of time logging software, not unlike what accountants use. It is going to be extremely important that you are able to account for your time spent on projects.
Choose an office location with good accesibility to your market, and good parking - builders have a habit of dropping in a lot, which is a good thing and encourages business but you will need somewhere to put their cars, clients cars and yours.
HTH - If I can be of anymore assistance, please let me know!