I think its not legal to carry any weapon for self defence. If you're caught carrying a blade in public you can be arrested if the police officer sees fit to.
from the offensive weapons act
"
9. (1) Subject to subsections (2) and (3), where a person has with him in any public place any knife or any other article which has a blade or which is sharply pointed, he shall be guilty of an offence.
(2) It shall be a defence for a person charged with an offence under subsection (1) to prove that he had good reason or lawful authority for having the article with him in a public place.
(3) Without prejudice to the generality of subsection (2), it shall be a defence for a person charged with an offence under subsection (1) to prove that he had the article with him for use at work or for a recreational purpose.
(4) Where a person, without lawful authority or reasonable excuse (the onus of proving which shall lie on him), has with him in any public place
( a ) any flick-knife, or
( b ) any other article whatsoever made or adapted for use for causing injury to or incapacitating a person,
he shall be guilty of an offence."
full link
http://www.acts.ie/zza12y1990.4.html#zza12y1990s9