The main source in the GRO research room are the Index books, the actual registers are not available to the public. The Index books contain the same details as the index available online on familysearch - see
BMD Index i.e. name, year/quarter, registration district, volume number & page number. When searching you can request 5 years of index books at a time (for either births, marriages or deaths) - there is a charge for this, I think €2.
Once you find possible record for the event you are looking for you can then fill out an order form with the references and await a research cert (€4). There is a maximum of 5 per person/per day.
The room can be busy at times, so it's a good idea to arrive as they open at 9:30. Certs usually take about 10 or 15 minutes to arrive... although these can take longer when they are busy.
Shane