The 30-year-old striker has joined Argyle on a month's loan from the Bees and will be available to manager John Sheridan up to and including the home match against Northampton on November 2.
He is also eligible to play in Tuesday’s Johnstone's Paint Trophy tie at Swindon Town.
Paul is a graduate of Norwich City's successful youth academy but left Carrow
Road in 2002 having never made an appearance for the club.
He joined Scunthorpe United and, over the next three years, scored 34 times in 116 appearances.
In the summer of 2005, Hayes signed for Barnsley and completed two seasons at Oakwell before returning once more to Glanford Park.
Paul's second spell was just as successful as his first as he made another 150 outings, weighing in with 40 strikes.
However, in 2010, he swapped Lincolnshire for Lancashire, joining Preston North End for a season.
Having enjoyed a short loan return to Barnsley, Paul joined Charlton in June 2011 before signing for Brentford at the beginning of last season.
He turned out 32 times for the Bees last season and also played 11 times in a loan spell at Crawley.
He returned to Griffin Park in time to make two substitute appearances in the Bees' ultimately unsuccessful play-off campaign.
Paul will wear the number 19 shirt during his time at Home Park.