Hello Dalzelle, a tragic end to your great-uncles life. There are quite a few newspaper articles from that time about this incident. Here's one:
1921 Londonderry Sentinel - Saturday 29 January
YOUNG MAN SHOT DEAD
The announcement contained in Thursday's newspapers of the abominable murder of policemen in Belfast caused, as may be imagined, a tremendous sensation. It seemed almost incredible that in the very heart of the city, and within a few paces of the police headquaters and the Central Fire Station, such a deliberately planned and monstrous crime could be committed, and that the perpetrators could contrive (for the time being) to escape justice.
The constables as they lay in bed were shot by five men, who had remained in the hotel after closing hours. They knew the constables' bedroom, and coolly committed the terrible deed.
The murders had an almost equally tragic sequel in the early hours of Thursday morning. A young man named Michael Garvey, lodging at a house in Bray Street, Crumlin Road, was shot dead in bed by a party of three masked men, who opened the door with a latch key.
Garvey, who was about twenty-three years of age, and a native of Armagh, was a chemist's assistant, and is said to have been an ardent Sinn Feiner.
John M'Garvey, described as a barman, a fellow-lodger of the murdered man, was placed under detention. It was stated he did not return to the lodgings on Wednesday night, having been at a dance.
In the course of the interview, Miss Morgan said when the deceased man Garvey went home, he took a glass of milk and a piece of cake for supper. She retired to rest at 10:40, and was awakened by the sound of a key being turned in the lock of the door at a quarter to one. On hearing footsteps on the stairs she went on to the landing, but returned to her room on seeing three men, who were absolute strangers to her. One of the men wore a kind of veil, which effectively concealed his features. Two shots were fired simultaneously, and a few seconds later a third shot was discharged. She saw the men come out of M'Garvey's room, and called out—" You have shot the boy !" The men ordered her to stand back, and as soon as they had got clear of the house she went into M'Garvey's bedroom and realised at a glance that the young man was dead. She then went to the window and shouted—" Murder; they have shot the boy." Miss Morgan added that at the corner of the street there were six or seven men, who went away together. Deceased, who, she understood, came from county Armagh, had been lodging with her for the last eight or nine months. M'Garvey, Miss Morgan said, was out at a dance on Wednesday night, and that accounted for his absence at the time of the raid.
As regards the three constables, they were entire strangers to Belfast, and had come specially from the depot in Dublin in connection with an important investigation in Belfast. All three were young men. Constable Quinn was a native of Queen's County, and had been about three years in the R.I. C. Constable Heffron, who had about the same period of service, belonged to county Mayo. Both were unmarried. Constable Gilmartin, the wounded man, comes from Ballymote, county Sligo. It is believed that the three constables were followed from Dublin by the murder gang.
From an early hour on Thursday morning crowds flocked to the scene of Wednesday night's tragedy. The public-house premises, which were in the possession of the police, remained closed. The bodies of the murdered constables, as well as the wounded man, had been removed overnight.
Roddy's public-house--or the Central Railway Hotel, as it is called—is particularly well known to country people who come occasionally to Belfast, and especially to those who attend the markets. Situated at the corner of Oxford-street and Townhall-etreet, it is quite close to the Central Police Station, and also to Musgrave-street Barracks. In the spacious yard belonging to the premises farmers' carts by the score are accommodated on market days, and it is also a "pound" for stray animals.
Up to Thursday night no charge had been preferred against the two barmen, Vincent Waiters and Michael Murdoch, both of whom live on the premises. Both were, however, detained by the authorities pending inquiries.