"Dave's been brought in as cover for the injured Dave Nolan and we're delighted to have a player of his calibre in the squad. He's played here before so he's been able to fit straight into the setup and he gives us extra options in the second row," said Elwood.
Gannon has been capped for Ireland Schools and captained the Ireland Under 21 side that reached the 2004 World Championship final in Scotland. He was capped by Ireland A in 2007 against the New Zealand Maori and England Saxons.
Gannon said: "I'm excited to be back in Connacht and I'm looking forward to a big season ahead. It's been good seeing some old faces about, a lot has changed since I left and I've been impressed with the work ethic of the lads."