When the card refers to an "Eagle ally," the eagle ally doesn't need to be in play right? Does it at least need to be in your hand. I'm wondering since the card doesn't specify.

You can only target on table cards unless a card refers to other game areas. It is explained in the Advanced Concepts of the rulebook.

the eagles need to be on the table to tigger support. Still there is no tap effect or anything on these eagles. So while support can only be triggered once a phase … multiple copies can be triggered of the same eagle. So the goal of the eagle deck is to get eagles of the misty mountains in play, and have the other eagles go out of play and added as tokens to EotMM. This can add up very quickly.

So you have EotMM and it is say a 5(a)4(d) though you can get it way way higher… you can defend with it or attack, and still buff your attack hero with Support of the Eagles and your defence hero with its own Support of the Eagles card

This is a extremely powerful attachment.