I think Robb could have been a bit nicer.
The GRO in Dublin are not the most helpful of people but if you contact them via their website and give the details you have, you may be able to get her death certificate. If there is more than one Gertrude Coffey, you should get all of the certs. It will cost (I think) €10 per cert which can be paid by credit card.
As a further suggestion (and you might try this first), you could try the Irish Times or Irish Independent for death notices, which might give an address, which would cut down on the expenses. Genealogy is not a cheap hobby, as I have discovered in my many trips to the GRO! Best of luck.