Almost every defence in Irish criminal law works the same way on burden of proof. Once there is some evidence of the defence, the prosecution must disprove it beyond reasonable doubt; the accused carries only an evidential burden of putting it in play. That is the rule in The People (AG) v Quinn [1965] IR 366 Supreme Court (IE). Whether there is enough evidence to leave a defence to the jury at all is a question for the trial judge.
Insanity and diminished responsibility are the two great exceptions. On each of them the accused must prove the defence on the balance of probabilities. If you take one thing from this note, take that: the burden reversal is the first sentence of any answer on either defence, and it is the point most often got wrong.
Insanity under section 5
Section 5(1) of the 2006 Act puts the old common-law M'Naghten Rules on a statutory footing, with a distinctively Irish addition. Three things must be established: that the accused committed the act charged; that at the time they were suffering from a mental disorder; and that the disorder was such that the accused ought not be held responsible, because of one of three specific consequences. The court or jury must first have heard evidence of the accused's mental condition given by a consultant psychiatrist (s.5(1)). The Mental Health Act 2026 (No. 11 of 2026), ss.209, 212 and 213, amends ss.1, 5 and 6(1)(b) of the 2006 Act; those amendments are not yet commenced.
Mental disorder and disease of the mind
Mental disorder is defined in s.1 of the Act. It covers mental illness, mental disability, dementia and any disease of the mind — but it expressly excludes intoxication. That exclusion matters: a defendant whose reasoning collapsed because of drink or drugs is arguing intoxication, not insanity, and the rules are completely different.
"Disease of the mind" is a legal concept, not a psychiatric one, and it is far broader than students expect. It reaches any condition impairing reason, memory and understanding, and it need not be a disease of the brain at all (R v Kemp [1957] 1 QB 399 Queen's Bench Division (E&W)). On that reasoning epilepsy has qualified (R v Sullivan [1984] AC 156; Bratty v AG for Northern Ireland [1963] AC 386 Eng), and so has hyperglycaemia arising from untreated diabetes (R v Hennessy [1989] 1 WLR 287 Eng) — because the malfunction there flows from the disease itself.
The three defect-of-reason limbs
It is not enough to have a mental disorder. The disorder must have had one of three effects, and you should name which one you are relying on:
- The accused did not know the nature and quality of the act. This is about the act's physical character, not its morality (R v Codere (1917) 12 Cr App R 21 Eng).
- The accused did not know that the act was wrong. In the English cases "wrong" means legally wrong: an accused who knew the act was against the law is outside this limb even if they thought it morally justified (R v Windle [1952] 2 QB 826 Eng). The Irish position is less settled: s.5(1)(b)(ii) says only "wrong", and the Supreme Court in Doyle v Wicklow County Council spoke of understanding the act's wrongfulness "morally and legally".
- The accused was unable to refrain from the act. This is the irresistible impulse limb — the Irish addition.
Irresistible impulse is where Irish law parts company with the original M'Naghten Rules. It was recognised in Doyle v Wicklow County Council [1974] IR 55 Supreme Court (IE) (Supreme Court), and it is genuinely a control-based test rather than a knowledge-based one. But it is policed strictly: the impulse must have been irresistible, not merely unresisted. Evidence that the accused could have stopped but did not is fatal.
The special verdict
Success does not produce an ordinary acquittal. It produces the special verdict of not guilty by reason of insanity — an outcome that acknowledges the accused did the act while withholding criminal responsibility for it.
Diminished responsibility under section 6
Diminished responsibility is a much younger defence in this jurisdiction, and it is entirely statutory. It did not exist at common law — that was settled in The People (DPP) v O'Mahony [1985] IR 517 Supreme Court (IE). Section 6(1) of the 2006 Act created it, and s.6(2) puts the burden on the accused: "it shall be for the defence to establish" it.
The structure is deliberately parasitic on insanity. The accused must show that they did the act, that they were suffering from a mental disorder, and that the disorder was not such as to found the insanity verdict but was such as to substantially diminish their responsibility for the act. Think of it as the half-way house immediately below insanity: the disorder is real, but not disabling enough to defeat responsibility altogether.
Two limits are worth stating expressly. First, it is available only on a murder charge; on any other charge a mental disorder short of insanity goes to mitigation of sentence, not to guilt. Second, the verdict is not an acquittal: the accused is not guilty of murder but guilty of manslaughter. Like provocation, it operates even where the accused genuinely intended to kill — it concedes the mens rea and attacks responsibility instead.
How the two defences actually differ
They share a statute, a defined term ("mental disorder") and a reversed burden, which is exactly why candidates blur them. The differences are clean once you separate three questions.
What charge? Insanity can be raised on any charge; diminished responsibility only on murder. What verdict? Insanity gives the special verdict; diminished responsibility gives a manslaughter conviction. How severe must the disorder be? Insanity requires a disorder producing one of the three defined defects of reason; diminished responsibility requires a disorder that falls short of that but still substantially diminishes responsibility. In a murder problem it is entirely normal to run them in the alternative — insanity first, diminished responsibility as the fallback — and a good answer says so.
The automatism boundary
Insanity is also the destination of one branch of automatism, the plea that the accused's conduct was involuntary because the mind was not controlling the body. What decides which branch you are in is the cause of the malfunction. An internal cause — a mental disorder — is insane automatism, so you are back in s.5 with the accused bearing the burden and the special verdict at the end. An external cause — a blow, an anaesthetic — is non-insane automatism, which produces a straight acquittal with the prosecution bearing the burden.
The classic illustration is diabetes, and it cuts both ways. Hypoglycaemia brought on by taking insulin is externally caused and grounds non-insane automatism (R v Quick [1973] QB 910 Eng); hyperglycaemia from failing to take insulin flows from the disease itself and is insane automatism (Hennessy Eng). Whichever branch applies, the accused must show a total destruction of voluntary control — impaired or reduced control will not do (O'Brien v Parker [1997] 2 ILRM 170 High Court (IE)). And where the external cause is drink or a dangerous drug, you leave automatism altogether and apply the intoxication rules.
Related Criminal Law notes: murder vs manslaughter · self-defence · all free Criminal Law notes.
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Who has to prove insanity in an Irish criminal trial?
The accused, on the balance of probabilities. That reverses the ordinary rule in The People (AG) v Quinn, under which the prosecution must disprove a defence beyond reasonable doubt once there is some evidence of it. Diminished responsibility carries the same reversed burden.
Is diminished responsibility available on charges other than murder?
No. Section 6 of the Criminal Law (Insanity) Act 2006 makes it a partial defence to murder only, reducing murder to manslaughter. On any other charge a mental disorder short of insanity goes to sentence rather than to guilt.
Does being drunk count as a mental disorder?
No. The definition of mental disorder in s.1 of the 2006 Act expressly excludes intoxication. A defendant relying on drink or drugs is running the intoxication rules, which turn on whether the required intent was in fact formed, not the insanity rules.
Is irresistible impulse part of Irish law?
Yes. It is the third limb under s.5(1) and it was recognised in Irish law in Doyle v Wicklow County Council, even though it formed no part of the original M'Naghten Rules. The impulse must have been genuinely irresistible, not merely unresisted.
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