The Lisburn Historical Society published a "Letter from Samuel Pelan to his son Thomas Rutherford Pelan" [1]. I have researched additional genealogical information relating to this story as well as much corroborating evidence - including the tale of Ralphs & Pelan - which may appear here at some point. If you believe you are related to him, please let me know.
In the story, Samuel writes;
" It happened that on that day I did not return from dinner until near 8 o'clock when on going down the yard it being then twilight, Hamill pounced on me suddenly and unperceived and made a blow at me with the tool above mentioned with the intent to cut off my head"
...
"In the meantime the rascal was seized and sent to Downpatrick gaol where he lay until the following assizes in July, when he was sentenced to eighteen months' imprisonment and after his liberation he went to America and I never heard anything more of him."
The Down County Museum has the corresponding court records (Assizes of August 1820):
" James Hamill, indicted, under Lord Ellenborough's act, for cutting and stabbing Samuel Pelan, on the 11th April last at Comber and also an assault on said Pelan. Guilty- of an assault - to be imprisoned for 12 months and to give security to keep the peace."
Also reported in the Belfast Newsletter.
References
Lisburn Historical Society Journal, 5 (1984), 29-41.