Penalty try.

Northampton Saints put the ball into a scrum five metres from the Munster line. They win the ball, hold it at No.8 and shove Munster back quickly towards the goal-line.

The ball does not reach the goal-line because Peter O'Mahony (7) of Munster puts a foot in and kicks the ball away.

If it is still a scrum and the ball is under Roger Wilson's feet, then O'Mahony infringed. He did it deliberately. If he had not infringed it is almost certain that Northampton would have scored. That makes the penalty try a clear and obvious decision.

Should not a warning precede a penalty try? After all this had not occurred previously in the match.

No. A penalty try is for a one-off, stand-alone situation. It is not a punishment for an accumulation of infringements?

Should the referee have sent O'Mahony to the sin bin?

According to law, he should have.

Law 10.2 UNFAIR PLAY
(a)  Intentionally Offending. A player must not intentionally infringe any Law of the Game, or play unfairly. The player who intentionally offends must be either admonished, or cautioned that a send off will result if the offence or a similar offence is committed, or sent off. Sanction: Penalty kick
A penalty try must be awarded if the offence prevents a try that would probably otherwise have been scored. A player who prevents a try being scored through foul play must either be cautioned and temporarily suspended or sent off.