Tom Barry in Guerilla Days mentions Monaghan a number of time. He says Monaghan was born in Scotland of Irish parents and deserted to the IRA with his rifle. Ambushes & Armour say Monaghan was a Royal Engineer attached to the Cameron Highlanders. Liam Deasy in Towards Ireland Free mentions him extensively. Everyone seems to agree that Peter Monaghan was not his real name
Deasy in "Towards Ireland Free"goes into some detail on Monaghan. It was 24th Dec 1920 at 11pm when 2 British deserters were noticed , and actually called at Deasy's house for food and cigarettes. His mother supplied them with both, but as the deserters left the house they were arrested by the IRA. They held the two at Knockpolgue in Kilbrittain area. What struck Deasy as odd was that the two men had virtually nothing in common. Monaghan was a well educated man, far above the standard of the average British soldier. The other man gave his name as Tommy Clarke and was uneducated
Monaghan spoke with the accent of a well educated Scotsman. He said they had left Ringaskiddy 3 days before and had travelled across country to try to meet with and to offer to help the IRA. Monaghan claimed that he had been a Captain in the RE during the war. On being demobilised he discovered there was nothing for him back in Scotland and had joined the Cameron's under the assumed name of Peter Monaghan. Deasy says that normally they would have shot the men as spies, but they were intrigued by Monaghan's claim to have been a mining engineer before the war, and to know something about explosives. For that reason they decided to take him on to make mines for them. No mention is made of what happened to Clarke- WS1621 says he stayed in the area working with farmers but never took part in any IRA activities
WS0540 The next day we heard that the body of a loyalist named Bradfield was found stretched at the crossroads where we had parted from the Column the night before. What had happened was this. When the Column came into the area it dispersed to different houses to secure food and shelter and some of its Officers entered Bradfield's home and asked for a meal. Contrary to the reception they expected they were welcomed heartily by the owner and invited to partake of whiskey. For some little time they were puzzled and then discovered that due to their appearance trench coats and bandoliers and semi-uniform look, and also to the accent of Peter Monahan, an Irish Scot, they were mistaken for Auxiliaries. Bradfield unbosomed himself to them and gave all the information he could of rebels and rebel houses in the district. He asked them why they did not get Frank Hurley (my brother). He was told they had him already and one of the 'Auxiliaries' stepped outside and arranged for Frank to be brought along. He was duly escorted in without trench coat, arms or equipment and, to all appearances, a prisoner. The delighted Bradfield immediately identified him as an important rebel, deserving of all he obviously hoped was coming to him. It was not long, however, till he was undeceived as to whom his guests were and was told of what his fate was to be. When later on I was in the field where the Column had re-assembled, the informer must must have been there too, and when I accompanied the marching Column part of the way Bradfield must have been in its midst walking to his death.
The balance of probability is that Monaghan was a British spy. If he was he was a very brave man.