The solution is obvious.
Your dragon was only a protector of the seal...
and there will be another.
To summon Erinn, the seal itself must be shattered.
You must reach Elchulus the everlasting god of time, and you must destroy him.
He will be well-guarded. Morrighan sacrificed herself to lock him away, and she will not have done so in vain.
It's all in the scriptures. Were you only to study them...
Your path to the seal may be clear now, though I would not be so sel-assured of an easy journey, were I you.