Cahill has previously played for Millwall, Norwich and Gillingham and has been brought in as an injury replacement for Shane Huke.
The Mariners have released Jamaica international Wolry Wolfe, who failed to make an impression during his time at the club.
“While it is unfortunate Wolry Wolfe’s time here wasn’t as successful as he or us would have liked, it gives Lachlan a great chance to come in and prove to us what he has got,” Mariners executive chairman Lyall Gorman said.
“Lachlan is held in very high regard by a lot of this country’s leading football people, and this is a great chance for him to prove himself at this level.”
