During the past month business at the Larne Shipbuilding Works has been exceedingly brisk, and the carrying out of new orders is still proceeding apace. […] There was launched on the 19th inst one steel motor barge, 70 X 16 X 7 feet, and fitted with 40 BHP Bolinder engines, to consume crude oil. The barge was built to the order of Messrs E Dowley & Sons, Ltd, of Carrick-on-Suir.
[…] The motor engines are installed by Bright’s Patent Pulley Co, Portadown.
Larne Times and Weekly Telegraph
22 February 1913
I don’t have the dimensions of the Big Knocknagow, but 70 X 16 is larger than the Little Knocknagow, so I suspect that this shows that the Big Knock was built in Larne and launched in 1913.
No doubt information about the origins of the Little Knock will turn up at some stage.
Irish waterways history 