"There was a lot of interest in Tadhg," boss Ian Sampson told the club website after landing the 25-year-old.
"He performed very well last season and to score nine times for a struggling side in less than half a season shows the potential he has."
Irishman Purcell made an immediate impact in English football after signing for Darlington on a short-term deal in January.
He scored the opening goal of the Quakers home game against Northampton in January.
"We have had to be patient to get our man, but we have beaten off a lot of competition to land him," said Sampson.
"He played well against us and when we knew he was a free agent this summer, we were keen to bring him in.
"He is a good finisher, has a lot of physical strength and understands the game."