Donnelly, 20, signed for the Pilgrims on Thursday after choosing them ahead of Scunthorpe United.
The South Coast side will pay the Unibond League side Skelmersdale an undisclosed fee for the forward.
"It feels great to not have to worry about work and just do what I love doing," he said.
"I used to work in a warehouse and get up at five in the morning, so I would train on a Thursday and play matches on Tuesdays and Saturdays. It is life-changing.
"The manager here has told me to fight for my place in the first-team, and show what I can do."