He wandered the palace gardens for about 10 minutes before his arrest on Wednesday evening. He had been unarmed, Westminster Magistrates` Court heard.
Hennessy pleaded guilty and was jailed for four months on Friday.
He admitted one count of trespass on a protected site and one count of criminal damage.
`Is Ma`am in?`
He had cut his right hand as he climbed over the top of the wall, which is between 8ft to 10ft high, and set the alarm off.
Prosecutor Tom Nicholson told the court that he had repeatedly asked, "Is Ma`am in?" as he was detained.
The Queen was in residence at the time, with the Duke of Edinburgh and the Duke of York.
Hennessy told police he had "walked through the gardens admiring the view".
Sikander Choudry, defending, said the unemployed stonemason had drunk "four or five cans of cider" before the incident.
He went to meet a friend in central London, and on his way to Victoria had another drink in a pub before deciding to climb the palace wall, using a nearby tree.
Mr Choudry said: "Mr Hennessy did not have any malicious intent towards the Royal Family - that`s not the reason he went to Buckingham Palace."
Iron bar attack
Recalling Hennessy`s previous conviction for murder, Mr Nicholson told the court that Hennessy had been 17 when he was accosted by a homeless man who had asked for money and knocked food out of his hand.
"Hennessy became angry and attacked him with an iron bar, fracturing his skull into small pieces," Mr Nicholson said.
Hennessy, who had been sniffing butane gas before the attack, then jumped on the man`s head, killing him.
He was convicted in 1993 at the Old Bailey and released from prison in 2002. The probation service stopped monitoring him in 2013. He also had a conviction for shoplifting in 2016.
Chief magistrate, senior district judge Howard Riddle, jailed Hennessy for four months for trespassing and two months, to run concurrently, for damaging the wires of the alarm system, to the value of £2,000.
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