What's Fiacra? I know it's a giant eagle, but is there anymore information on it?

He is "The Northwind." He is almost certainly one of the many Beasts (such as Dragons) who were created as weapons with which to fight the Godswar.

Ok, I knew Mulcarn created him, then had to seal hi away with the signing of the Compact. I just diddn't know if there was any more information on Him.

Not really. Just that I believe that Kael said that unleashing such a "pet" upon creation does not constitute a breach of the Compact.

Well, it dosn't really matter if it was a breach in Fiacra's case, Mulcarn broke it atleast four times. And he'd already entered creation when he released him.