A jury has begun deliberations in the double murder trial of a man who denies murdering two elderly brothers in their Mayo home. 

Mr Justice Paul Coffey has charged the Central Criminal Court jury in the trial of 30-year-old Alan Cawley.

Mr Cawley of Four Winds, Corrinbla, Ballina, Co Mayo has admitted killing 69-year-old Thomas Blaine and 76-year-old John (Jack) Blaine.

However, he has pleaded not guilty to murdering them on 10 July 2013 at New Antrim Street in Castlebar.

The jury must consider if Mr Cawley had a mental disorder that diminished his responsibility for killing the two men. 

Both men had special needs and were found dead by their home help.

They had been beaten prior to their deaths.

Mr Justice Coffey explained that whether the accused had caused the men's deaths was not an issue that would trouble the jurors.

"In this case, the defence have raised a partial defence, known as diminished responsibility, and that is the nub of the case that you must consider," he told them.

"You cannot proceed to convict unless and until you have considered that defence."

He explained that there had to be evidence that, at the time of the killing, Mr Cawley was suffering from a mental disorder, which substantially diminished his responsibility for his acts.

Mr Justice Coffey read out the explanation for mental disorder given in the Criminal Law Insanity Act 2006: "Mental disorder includes mental illness, mental disability, dementia or any disease of the mind but does not include intoxication."

"The word, 'includes', suggests that the psychiatric diagnoses listed in the Act is not exhaustive," said Mr Justice Coffey.

He noted that the defence case was primarily advanced through the evidence of a psychiatrist, who said that Mr Cawley had three mental disorders when he killed the Blaines.

The doctor testified that these were Attention Deficit Hyperactivity Disorder (ADHD), Antisocial Personality Disorder and Emotionally Unstable Personality Disorder, all of which he said caused him to be more impulsive.

The psychiatrist said that all three were mental disorders clinically and also within the meaning of the Act, constituting diseases of the mind.

The judge noted that the prosecution had called a forensic psychiatrist in rebuttal.

"Her opinion was that he does not suffer from adult ADHD and was not so suffering at the time," he recalled.

"She accepts he has the two personality disorders," he continued.

However, he noted her opinion that they did not constitute mental disorders within the meaning of the 2006 Act as they did not impair capacity.

"I must say to you that the 2006 Act says nothing at all about capacity," he said. "You are at large in deciding if they are mental disorders within the meaning of the Act."

He said that, if satisfied that the accused had a mental disorder, the jurors must then decide if it substantially diminished his responsibility for the acts. This had to be done once they had subtracted any element of intoxication.

"Whether it does so or not is a matter for you," he said, noting that the defence's psychiatrist had said they had played more than a trivial or minimal role.

He said their common sense would tell them the meaning of 'substantial'. He said that it did not mean 'total' or 'trivial', but something in between.

He told them that if they were satisfied of the defence on the balance of probabilities, they must return a verdict of not guilty of murder, but guilty of manslaughter. He explained that, if not satisfied, they should find the accused guilty.

The four women and eight men had spent just over half an hour considering their verdict before being sent home for the evening. They will resume their deliberations tomorrow morning.