William Straw - whoever he was

The American commission reported that "We learned from testimony regarding the killing of John Sherlock of Skerries, an Irish Republican, by British agents, that one Penstraw, who is alleged to have acted as guide to the British at the sack of Balbriggan, had been assassinated there about a month later. Altogether we have been able to trace thirty assassinations of members of the Imperial British forces, presumably at the hands of the Irish (five accused with Swanzy of the murder of Lord Mayor MacCurtain, Smyth, fourteen officers in Dublin, two at Thurles and the others noted).
But "Penstraw" is a surname that does not exist anywhere.

But I did find this here . Penstraw appears to have been shot and buried at Skerries and the bodied surfaced a month later. The shooting of Sherlock appears to have been a retaliation for Penstraw's death. So he left the town the day after, and he was in Skerries. The report was that the Skerries Volunteers put him out of Skerries. That was the report. That was about three or four days afterwards. About a month after that, there was a body got about eight miles away in a ditch. It turned out to be Penstraw. He was not buried right. There was heavy rains on. There was some young lads in the ditch getting blackberries, and they found the body. Well, then, he was identified. The police were very active, and he was identified as Penstraw.

WS 1043 by Joseph Leonard of the local IRA at the time sheds light on the execution.

... but this was a stranger who appeared quite suddenly and who gave no plausible account of himself. He became known as Jack Straw, whether this was the name he gave himself or whether it was an appellation applied by the country people I do not know, but he was viewed with the suspicion that any such stranger excited at the time. After the sack of Balbriggan the story of his appearance there in company with the R.I.C. began to circulate and Volunteer Intelligence Officers, began a check-up, but now it seemed the individual had disappeared. He was reported again in the district about a fortnight later, but no responsible Volunteer Officer could manage to get sight of him until one day Dan Brophy happened to be in Lusk on business - he was at the time employed as the driver of the motor van belonging to the Swords Co-operative Society - and he was told by one of the local Volunteers that the now notorious Jack Straw had passed through the town some ten minutes earlier heading towards Corduff. Brophy did not know Jack Straw by sight and so he took his informant with him in the van and in due course passed and had identified to him the wanted man. Brophy drove on to Corduff where he called to the house of the Kelly family. Joe Kelly was the Brigade Intelligence Officer. There he found his brother Tommy Kelly, whom he informed of the situation and obtained a weapon and some cord to tie up the intended prisoner, and returning the road he had come he accosted the supposed beggar and invited him to accept a lift in the van, which he did. The prisoner was taken to Dempsey's mill at Grace Dieu, where Brophy handed him over to Willie Dempsey with very strict injunctions as to his. safe custody. Willie, I believe, took this very seriously, and to ensure that he would not escape tied him to one of the heavy stanchions which supported an upper floor of the mill. Meanwhile Brophy made contact with the Brigade Commander and a formal courtmartial of the prisoner was arranged for that night at the mill. Brophy told me afterwards that he found it difficult to convince the Brigade Commander who presided on the courtmartial that the evidence against the prisoner was sufficiently conclusive to warrant his conviction as an enemy agent, but the court finally agreed on a verdict and Straw was condemned to death. The execution was carried out later by Brophy and Joe Kelly, and in the end Straw admitted that he was in fact a British Intelligence Officer and died bravely. When Brophy asked him whether he had anything to say before he died, he stood erect and folded his arms replying: "No, when I undertook this mission I was fully aware of what the end might be and now I accept my fate without complaint". Jack Shields, another of the Ballyboughal Officers, had been instructed to prepare a grave in a field on the hill north of Ballyboughal near the "Nag's Head", but when the execution party arrived late at night they found the tools were there but no grave dug. The body was therefore placed in a dry ditch and the earth from the bank above thrown in on it. Cattle in the fields, however, stamped over the fresh earth within the following day or two and so exposed the body, which was removed by a searching party of Black and Tans a little later.

WS 1398 has less An ex-British army soldier, who went by the name of Jack Straw - which was not his real name, guided the Tans around the town and pointed out to them the houses to burn. Straw was not a native of the area. He was subsequently arrested by us for the part he played that night, and was duly executed.

WS 1399 is not any more forthcoming . When Balbriggan was burned by the Tans consequent on the shooting of a couple of that force in a publichouse, there was a man named Hemstraw who was either a serving or an ex-British soldier, who led the Tans around and pointed out the houses of Volunteers; and Sinn Féiners to them. We were very anxious to pick up this man and eventually did. so. He was courtmartialled and shot

So putting it all together what can we say

It is about 6 miles from Balbriggan to the body

Regiment - If it was not Argyll & Sutherland Highlanders, what could have been, most like runner so far is

Relatives in Balbriggan

Black and Tans

The death certificate for "William Straw" found dead in the shallow grave at Bettyville 21 Oct 1920

Thompson's Intelligence men