How does one get the treasure from the leprechaun? also where is the portal where kerah is in order to get the obelisk? too revive the ashes?

You'll have to place Kerah's statue on the alter to get the column to move up to where she is. The portal and the leprechaun treasure are both there.

You need to do the ash thing after visiting Kerah but look arounf for the leprechaun first as you won't be able to get back. It's a bit more involved than just getting the obelisk.