To be desired is to be wanted by someone or something. To be desirable is to have a quality that makes the object desired. You can desire an apple. The delicious apple is desirable to you.

We don't use the word 'undesired' . Something is desirable if you desire it. If it is not to be desired, it is 'undesirable'. He has an undesirable reputation (a reputation that you would NOT desire for yourself). The apartment is located in an undesirable part of the city. You would never say - I undesire her! Although, grammatically, you could say "I find her undesirable".