is there a way via api to trigger a map reduce script if yes any docs would be help full meaning to create a map/reduce task via api

use task.create to create a task.MapReduceScriptTask and submit it

This is a external application using suitetalk

use a user event script to do the tasking

How does the user event get triggered

on save of a record

Using userscript will trigger numerous of times if you have volume of records creation. I would suggest you to do it through suitelet based on

suitelet creating an external URL?

a restlet is a good option if you wanted options that arent suitetalk related

so if I go the restlet route that will give me an external URL ?

You could use Restlet but, it is going to be authenticated, if you want to bypass authorization then go through SuiteLet with external URL

no i need auth i cant have a url exposed to trigger script

Then you could go through RestLet

would you have the link to sample reslet with auth