Explain the irony Act I, scene 2, lines 51-53 of "Macbeth".what is the irony and explain it.

I am not very sure if your question relates to the following lines of Macbeth act 1 sc.2 :

" Norway himself

With terrible numbers,

Assisted by that most disloyal traitor,

The Thane of Cawdor, began a dismal conflict;

Till thatBellona's bridegroom......................"

In these lines, Ross reports to King Duncan how Macbeth accosted the invading Norwegian army camped in Fife backed up by the Thane of Cawdor. Macbeth proved himself a great warrior-- as if the groom of the goddess of war, Bellona--clad in armour proof against all offensives. The irony lies in the fact that Macbeth who proved himself so heroic, noble & loyal as opposed to Cawdor who was a traitor, was going to be the killer of the king and a vile usurper of the throne of Scotland. The fall of Macbeth was the fall of the most trusted and the most widely admired man, just as Satan who fell from Heaven was also the brightest of all angels in Heaven.