Amongst other things, I'm a mortgage broker so this isn't exactly an unbiased view, but I'll try to be balanced.
A good mortgage broker should tell you from the outset which lenders they deal with. It's on their Terms of Business letter anyway - you should receive that early on. So a mortgage broker will offer you a choice between the products of the various lenders they deal with. Or you can do that research yourself by checking out the rates and products of all the lenders.
A good mortgage broker should be able to give you an indication from the outset of what level of mortgage you might qualify for, or tell you from the outset that you won't get the mortgage you're looking for. That said, brokers are not infallible in this regard and it's quite difficult even for a broker to predict what way a marginal application is going to go. Lender criteria are changing regularly at the moment.
A good mortgage broker can give you opinions about lenders that the lenders won't give you themselves. For example, Knuttell mentions Permanent TSB above. They have some excellent fixed rates at present, but I'd be VERY wary of recommending them to a client given what they did to existing customers on Standard Variable Rates, which has its own forum here on Askaboutmoney. If you approach PTSB directly, they're unlikely to highlight this kind of thing.
If your mortgage broker is also a life insurance broker and/or house insurance broker, they can get you a choice of quotes on the required insurances. Most lenders are tied to just one source of insurance products, which may or may not be the most competitive. That said, you can shop around for the insurances yourself also if you wish.
As Knuttell mentions, some lenders don't deal with brokers and a good broker should make you aware of this from the outset. For example, Ulster Bank, National Irish Bank and Bank of Ireland don't deal with brokers, although ICS do and ICS is a subsidiary of Bank of Ireland.
I think you're right seeking a recommendation from someone who has actually used a particular broker. Have you friends or family who can recommend a broker to you?