Self-Defense or Undue Force.

What constitutes undue force?

If someone breaks into your home in the middle of the night and you practically decapitate them with a golf club you keep beside the bed specifically for protection, would that be considered undue force?
If you are out running/walking/getting the papers and someone grabs or tries to rob you and you almost break their jaw with an elbow, is that undue force?
If you catch someone trying to break into your car and you punch them in the side of the head hard enough to knock them down is that undue force?
If you come home from being out and find an aggressive drunk asleep in your front porch and the only way to get them off your property-and stop threatening your family- Is to almost break a mop handle over him, is THAT undue force?

Clearly I am confused as to what constitutes undue force and what constitutes self-defense. All of the above strike me as perfect acts of self-defense and protection of self and property, but according to a gal I was talking with over the weekend the situations described should end with nothing more than a call to the Gardai. She said anything more could be construed as 'undue force' and might very well end up with the self-defender being dragged into court!
(And since she's a barrister I'm inclined to believe her.)
But I can't get the conversation out of my head. I can't believe the law could be that skewed.
So how about it? Undue force or reasonably reaction?