There are not really any extra benefits in going directly to the lending institution yourself - in fact the broker probably has contacts at the banks and can push the mortgage through if the multiples are tight.
There should be no cost to you for using a mortgage broker so if they want to charge a fee then don't use them. They are paid a percentage of the loan value from the institution (1% I think).
This is simply not true in all cases.
AFAIK, National Irish Bank do not use Brokers to sell their Mortgage products, so no Broker can actually arrange a Loan for you with NIB at present, yet they constantly offer better deals than most other lenders for particular products.
If you use the services of a Broker, you should also check with NIB, to see of they can match or better whatever best offer your Broker can get.
IMHO, there is nothing wrong or underhanded in this, especially if you let the Broker know what you are doing.
Also, some Brokers do actually charge a fee for service, and then refund the bounty from the lender to the client, i.e. REA.
They will claim that this protects clients from unscrupulous brokers who may recommend a lender which pays the biggest bounty to the Broker, which may not necessarily be the best deal for the client. I would have a lot of sympathy for this model.