you really should speak with your local planner, you'll have the
best chance of getting permission for this if you can demonstrate
a need for the area like a 'granny flat'. Secondly the money you
spend on a professional making the planning application in my view
is well worth it, you'll get a professional to build your cabin and likewise
the drawings should be treated in a similar fashion. remember your planing with
the commencement notice will take you minimum 3.5 months and add
the time it'll take you to prepare a
valid planning application could
take a couple of weeks and if you make a mistake they'll return the
whole appliocation and you'll have to re-lodge it.
If you're hell bent on doing it yourself then go to the planning dept
and ask to see a particular application (get the address first) that
was granted and look at it in detail and you'll see everything that
is involved.
look over the sections listed on the lefthand side of the page here for
all that is involved, this is fingal's website but it all comes from the DOE.
[broken link removed]
good luck