NIB do not offer their products via brokers.
NIB only offer the best deal in the market for certain types of mortgages to qualifying borrowers. If those are your requirements, a broker cannot arrange the best deal for you.
For all other mortgage types, a good broker can get you the best deal.
It has been said in this thread that you can haggle yourself with a lender to get a better deal directly. Sometimes you can. This will only work if your application is strong. Don't bother trying it with a small mortgage or one where you're approaching the limit of your perceived affordability. If your application is strong, a good broker may be also able to haggle with lenders to get you a better deal.