The 23-year-old, who netted the winning goal in the Pilgrims' opening pre-season friendly against Grimsby Town, has linked up with Craig Elliott's side following a successful trial spell.
The former Cabinteely and Frickley Athletic defender sat out the Lincolnshire Senior Cup tie against Scunthorpe United on Wednesday with a knock but is expected to be fit to face Grantham Town on Tuesday.
"Karl has shown a real determination and hunger to want to sign for Boston United," manager Craig Elliott told bufc.co.uk.
"He has played at a very good level in Ireland and in his short stint at Frickley won numerous man-of-the-match awards.
"I have seen a lot of quality in his game and I am very hopeful he can adjust quickly to the demands of National League North."