TOPIC:

I'm trying to establish how John McCoy could have found his way from Northern Ireland to the Transvaal, South Africa.

His South African Death Notice doesn't give his parents' names. It states he was born in "North Ireland" and gives his occupation as Farmer.

His last will & testament states:
"I will ... the whole of my landed property consistent of two farms and also all my cattle wagons and other effects"
familysearch.org/ark:/61903/3:1:3Q9M-CS9...-Q?i=1189&cc=2520237

He died at the farm Rustplaats, near Lydenburg just 11 days after the siege that lasted eighty-four days. Casualties were four killed, including two volunteers, and nineteen wounded
samilitaryhistory.org/vol052mg.html

I'm quite curious whether he was involved in the siege and, if so, in what capacity i.e. British Forces / Boer Forces?

He was married to Susanna Elizabeth Johanna Davis whose father was, London born, James Davis and mother was, South African born, Maria Geertruyda Janeke.

You really need to know his place of birth and names of parents to make searches in Great Britain to be worthwhile, South Africa and in particular the Transvaal republic were a real melting pot and you find an awful lot of Irishmen within it during this period.
A great many had actually been deserters from the British Army who simply needed to disappear, absolutely no point whatsoever in going home, people were quite literally staving to death in the gutter in many parts of Ireland in those days.
Far better to at least try and establish a new life elsewhere and the Transvaal was the one place where Her Majesties Government would not be able interfere.
John McCoy is a common name, just going down the British Army route would be time consuming and quite possibly, not only inconclusive, but, completely fruitless given the lack of basic detail.

AnorakABO wrote: I'm trying to establish how John McCoy could have found his way from Northern Island to the Transvaal, South Africa.

His South African Death Notice doesn't give his parents' names. It states he was born in "North Ireland" and gives his occupation as Farmer.

His last will & testament states:
"I will ... the whole of my landed property consistent of two farms and also all my cattle wagons and other effects"
familysearch.org/ark:/61903/3:1:3Q9M-CS9...-Q?i=1189&cc=2520237

He died at the farm Rustplaats, near Lydenburg just 11 days after the siege that lasted eighty-four days. Casualties were four killed, including two volunteers, and nineteen wounded
samilitaryhistory.org/vol052mg.html

I'm quite curious whether he was involved in the siege and, if so, in what capacity i.e. British Forces / Boer Forces?

He was married to Susanna Elizabeth Johanna Davis whose father was, London born, James Davis and mother was, South African born, Maria Geertruyda Janeke.

The article I linked mentions the 94th Regiment at Lydenburg, and the 21st Royal Scots Fusiliers at Rustenburg; I thought it worthwhile checking for a John McCoy amongst them. Also, important to note, that this is the First Boer War.

You stated in your original post that McCoy was a farmer, so if you go down the route I mentioned in my post, which was merely a suggestion of how he might have actually been in the Transvaal and nothing more than that, you still would need rather more information than you have.
If he had deserted from the British Army, he would not have been actually discharged, muster rolls of the period, are both incomplete and do not give the information you need to obtain proof.
If, on the other hand, he had been discharged at some point before the Transvaal War, you would need to know his next of kin and place of birth, in the hope his original papers would still be extant.

I was able to find:
First name(s) John
Last name McCoy
Service number 647
Rank Private
Unit or Regiment 2nd Bn 20th (East Devonshire) Regiment of Foot
HQ location King William's Town, South Africa
Year 1871
Country Great Britain
National Archives reference WO 12/3771
Period 01/04/1871-30/06/1871
Record set British Army, Worldwide Index 1871
Category Armed forces & conflict
Subcategory Regimental & service records
Collections from United Kingdom

The John McCoy I'm looking for had two children; the youngest of which was born 1873.