There's not a chance the bill will be in the hundreds im afraid.
The Planning and Development Act has been amended to allow for C-19 so statutory time frames etc have been extended. What this means is that that they will allow you additional time to prepare and lodge a Retention Planning Application.
Ok, fees, you really need to tell us what was quoted......but hundreds is not what it will be for the following reasons.....
The Newspaper cost will be at least €200.
The Planning Application fee is €102.
The OSi Maps are €70.
Then you add printing costs, lamination and erection of the site notice and the persons fee for doing the application.
DCC will require the full works of a normal Planning Application plus the details of the Retention and associated works.
This is what you are paying the Planning Agent for, Knowing how to put a valid PP together and lodging.
From doing these over the years, I would expect a bill of €xxxx.
Where in Dublin are you located?
Did you have any professional input into the garage conversion?
The first thing I do with these is to check the Parent planning to make sure there are no conditions de-exempting the exemptions, which yours obviously has. Reason I ask, is if they missed this, maybe they can arrange for the planning at a reduced cost.