What is the postal code for co Donegal?

Ireland is unusual in that it doesn't have formal postcodes.

Irish addresses are given in a reasonably straightforward format

Firstname Surname

Townland or Streetname

Smallertown

Biggertown

Co. Whatever

So, if you're posting to Co Donegal, simply place Co Donegal as the last line and once it reaches the postal service within the county, they'll send it onward. For example, to send something to the town of Letterkenny in county Donegal, a typical address could be like this:

Mr. A Smith

1 Main Street

Letterkenny

Co. Donegal

Ireland.

For an internet form looking for a postal code for anywhere in Ireland, just put IE as the code and that normally works.