Assuming you mean through their own in-goal?
Originally Posted by Casey Bee
If so, then its a 5m scrum with attacking team put in.
22.11 BALL DEAD IN IN-GOAL
(a) When the ball touches the touch-in-goal line or the dead ball line, or touches anything or anyone beyond those lines, the ball becomes dead. If the ball was played into in-goal by the attacking team, a drop-out shall be awarded to the defending team. If the ball was played into in-goal by the defending team, a 5-metre scrum shall be awarded and the attacking team throws in the ball.
The fact that it was from a penalty kick is irelevant.
If it is through their opponent's in-goal, then TigerCraig has the right answer
2. A blue defender catches the ball clean inside in goal but steps backwards through the dead ball line
Assuming the attacking team has put the ball in goal, then dropout 22m. Tha player stepping over the DBL has the same effect as if he had grounded the ball.
22.7 RESTARTING AFTER A TOUCH DOWN
(a) When an attacking player sends or carries the ball into the opponents’ in-goal and it becomes dead there, either because a defender grounded it or because it went into touch in-goal or on or over the dead ball line, a drop-out is awarded.