Novus Actus Interveniens in Irish Law: When the Chain of Causation Breaks

Origin and Adoption in Irish Law

The Latin maxim novus actus interveniens — "a new intervening act" — names a long-standing common-law principle of causation. In Ireland, the doctrine forms part of the body of negligence and tort principles inherited at independence and developed by the superior courts since. The Supreme Court has applied it across a wide range of fact patterns. Examples run from a ladder negligently left in the street (Cunningham v McGrath Bros [1964] IR 209) to a stolen motor car (Breslin v Corcoran & MIBI [2003] IESC 23).

The doctrine sits inside the wider law of causation. Once the plaintiff establishes that the defendant breached a duty of care, the next question is whether that breach caused the loss. Causation in Irish jurisprudence has two components: factual causation (the "but-for" test) and legal causation (sometimes called proximate cause or remoteness). Novus actus operates at the second stage. It accepts that, factually, the defendant's wrong set events in train. The question is whether a later, independent act so dominated the causal sequence that the defendant should not be held responsible for the eventual damage.

How Novus Actus Interveniens Works in Practice

Where a defendant has been found careless, the inquiry shifts. The court asks whether a later event so disrupted the causal sequence that the defendant should not bear responsibility for the damage. Three patterns recur in Irish personal injury practice.

Third-party acts

The classic pattern is a third party who, after the defendant's wrong, performs an independent act that directly causes the damage. The thief in Breslin [2003] IESC 23 who drove the negligently-keyed car into a pedestrian is the textbook Irish example. The defence asks: was the third-party act sufficiently independent, deliberate, and unforeseeable that it, rather than the defendant's earlier carelessness, should be treated as the operative cause?

The plaintiff's own conduct

A plaintiff who, after the defendant's wrong, takes a step that materially worsens the position can in principle break the chain. The Irish courts apply this category sparingly. Where the plaintiff's conduct is unreasonable but foreseeable, the typical analytical home is contributory negligence under section 34 of the Civil Liability Act 1961, not novus actus. Examples include refusal of recommended medical treatment, deliberate self-injury, or knowingly engaging in dangerous further activity after the original tort.

Natural events

An act of God — an extraordinary natural event — can in theory operate as novus actus where it is so unusual that no reasonable person could have foreseen it. In Irish personal injury practice, this category is rarely decisive on its own. Weather, terrain, and similar factors more often go to apportionment or to the scope of the original duty.

Leading Irish Cases Applying Novus Actus Interveniens

Three Supreme Court authorities define the modern Irish position. Each is decided on its facts but together they articulate the operative test and its limits.

One detail headnotes typically omit: in Breslin, Fennelly J. accepted that leaving keys in an ignition was both careless and a regulatory breach, yet still found no liability. The decisive analytical step was scope of risk. A careless car owner is not the insurer of the world against bad driving by an unknown thief. The leading case is often misread as a simple foreseeability test. The actual ratio in Breslin is narrower. What must be foreseeable is not merely some intervention by a third party, but intervention in the manner that produced the damage.

"It is not every novus actus which breaks the chain of causation."

per Kingsmill Moore J. in Cunningham v McGrath Bros [1964] IR 209, endorsed by Fennelly J. in Breslin v Corcoran [2003] IESC 23

When Novus Actus Does Not Apply

The doctrine is regularly pleaded and rarely decisive. Several patterns predict failure of the defence.

Where the intervening act is the very kind of thing the defendant should have anticipated. This is the Cunningham principle. If the defendant's want of care made the eventual harm a foreseeable consequence — even one performed by a third party — the chain holds. Irish courts apply this in cases of unattended hazards, items left in public spaces, and similar fact patterns.

Where the conduct is properly contributory negligence, not novus actus. Where the plaintiff's later conduct is careless but foreseeable, section 34 of the Civil Liability Act 1961 directs the court to apportion damages. The conduct is not treated as a complete causal break. The line is one of degree: the plaintiff must do something so independent and unreasonable that it is properly characterised as a fresh causal event. For a fuller treatment of apportionment in a clinical context, see our practical guide on apportionment in misdiagnosis cases.

Where the intervening act is reasonable mitigation. Where the plaintiff acts reasonably to mitigate the loss (for example, undergoing recommended medical treatment), even if that treatment turns out badly, the chain typically holds.

Novus Actus in the Medical Negligence Context

A recurring question in Irish personal injury practice is whether subsequent medical treatment can break the chain of causation between an earlier tort and the eventual injury. The textbook example: a road traffic collision causes injury; the plaintiff is taken to hospital; treatment is alleged to have been negligently performed and to have made matters worse. The original tortfeasor argues that the negligent treatment was a novus actus interveniens that confines their liability to the pre-treatment harm.

Irish courts approach this question cautiously. The orthodox position draws on UK authority such as Webb v Barclays Bank [2001] EWCA Civ 1141. Medical treatment must be so grossly negligent as to be a completely inappropriate response to the original injury before it severs the chain. Ordinary professional shortcomings during treatment are generally absorbed within the original tortfeasor's liability.

The line is shifting in the UK. The English High Court in Jenkinson v Hertfordshire CC [2023] EWHC 872 (KB) held that subsequent medical treatment falls to be analysed under the same standard as any novus actus interveniens. The court rejected a higher "gross negligence" threshold for clinical interventions. No Irish appellate court has yet adopted the Jenkinson approach, and the older "gross negligence" line remains the prudent working assumption for Irish pleadings. Practitioners running medical negligence claims in Ireland should be alert to a related tactical risk. Defendants will increasingly plead novus actus to direct the spotlight onto a treating clinician. See also our misdiagnosis and delayed diagnosis guides.

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