you mean Boromir....and his brother is Faramir...
the dream is:
Seek for the Sword that was broken:
In Imladris it dwells;
There shall be counsels taken
Stronger than Morgul-spells.
There shall be shown a token
That Doom is near at hand,
For Isildur's Bane shall waken,
And the Halfling forth shall stand.
ok...just incase...Imladris is another name for Rivendell
Morgul being the nazgul
Isildur's Bane being the One Ring.
It is a song in the book so thats why they would sing it....but it should be explained later in the council anyway