You are probably thinking of Gáirdín na n-ainmhithe which is what we call the Zoo, the animal's garden literally.
Nowadays its aceptable to say an zú
The children have a little rhyme
"Istigh sa zú tá mamaí kangaroo"
Na hainmhithe is the plural of the word ainmhí which means animal. The reason for the n- is that in the phrase you give the word ainmhí is in the genitive case and it is plural, the genitive happens when you have two nouns together the second one telling you more about the first. If you want more info on changes to the beginning of words after the article (an, na) and genitives, you are welcome to visit my site and search the grammer index.
http://www.iol.ie/~sefton Great to hear you are using Irish, you will be amazed how much you do know after short while.
Ádh mór leat!