Learn German: Ich finde es unerträglich, dass du mich ignorierst.

In this sentence, es acts as a placeholder (often called a Korrelat). The verb finden (when used to mean 'to consider something [adjective]') requires a direct object before you can add the adjective unerträglich. Because the actual thing you find unbearable is an entire clause (dass du mich ignorierst), you need es in the main clause to hold the grammatical spot of the direct object. You cannot leave it out here.

What happens to es if I put the dass-clause at the beginning of the sentence?

If you want to emphasize the action, you can move the dass-clause to the front: Dass du mich ignorierst, finde ich unerträglich. When you do this, the placeholder es must be dropped. This is because the subordinate clause now takes up the first position of the entire sentence, so the verb finde immediately follows it in the second position, and no placeholder is needed to point forward.

Why doesn't the adjective unerträglich have a declension ending?

Here, unerträglich acts as a predicative adjective describing the object (the situation represented by es). Because it stands alone and is not placed directly in front of a noun, it does not take any adjective ending.

Just as a refresher, why is the verb ignorierst at the very end of its clause?