Which part requires a license?

T-Maxx

Which part of a firearm (specifically pistol) is the actual part for which a license/permit is required? In other words, what exactly is the definition of a pistol?

Sparks

There isn't one.
From Section One:

“firearm” means—
(a) a lethal firearm or other lethal weapon of any description from which any shot, bullet or other missile can be discharged,
(b) an air gun (including an air rifle and air pistol) with a muzzle energy greater than one joule or any other weapon incorporating a barrel from which any projectile can be discharged with such a muzzle energy,
(c) a crossbow,
(d) any type of stun gun or other weapon for causing any shock or other disablement to a person by means of electricity or any other kind of energy emission,
(e) a prohibited weapon,
(f) any article which would be a firearm under any of the foregoing paragraphs but for the fact that, owing to the lack of a necessary component part or parts, or to any other defect or condition, it is incapable of discharging a shot, bullet or other missile or projectile or of causing a shock or other disablement, as the case may be,
(g) except where the context otherwise requires, any component part of any article referred to in any of the foregoing paragraphs and, without prejudice to the generality of the foregoing, the following articles shall be deemed to be such component parts:
(i) telescope sights with a light beam, or telescope sights with an electronic light amplification device or an infra-red device, designed to be fitted to a firearm specified in paragraph (a), (b), (c) or (e),
(ii) a silencer designed to be fitted to a firearm specified in paragraph (a), (b) or (e), and
(iii) any object—
(I) manufactured for use as a component in connection with the operation of a firearm, and
(II) without which it could not function as originally designed,

and
(h) a device capable of discharging blank ammunition and to be used as a starting gun or blank firing gun, and includes a restricted firearm, unless otherwise provided or the context otherwise requires;

About the closest you get to a definition of a pistol as opposed to any other kind of firearm is the definition of a "short firearm" in the Restricted Firearms SI (which is stolen wholesale from the EU directive on firearms):

“short firearms” means firearms either with a barrel not longer than 30 centimetres or whose overall length (including the length of any detachable component) does not exceed 60 centimetres;

Basicly, it's not like the US where the receiver is the defining component - Irish law is a lot more nebulous and inclusive in its definition of what a firearm is.

T-Maxx

Thanks for that.

So strictly speaking it would be illegal to even possess a single part, like a magazine or so?

Sparks

If it was an integral part and the firearm wouldn't fire without it, then definitely yes.
But it could be argued that even a part that the firearm could fire without could be illegal to possess - you'd probably have to have given the person making the charge a reason for that though (by which I mean that it's most unlikly someone would charge you for having a magazine, but if they wanted to, they could make such a case. Whether or not it'd hold up in court is another matter).