The 28-year-old former Blackrock College and UCD student made 55 appearances for Connacht, whom he originally signed for in 2005. He then moved to Air New Zealand Cup team Southland in 2009.
Gannon, who stands at 6ft 6in and weighs 17st, spent a season with Aviva Premiership side Exeter Chiefs in 2010 and it was during Exeter's recent pre-season game against Connacht at Dubarry Park that the province's head coach Eric Elwood caught up with him.
"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," admitted Elwood.
"He's played here before so he's been able to fit straight into the set-up and he gives us extra options in the second row."
Gannon played for the Ireland Schools and Under-19 teams and was the captain of the Ireland Under-21 side that reached the 2004 IRB U-21 World Championship final. He was capped for Ireland 'A' in 2007 against the New Zealand Maori and England Saxons.
Speaking about the one-year deal with Connacht, the Dublin-born forward 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. I'm looking forward to being a Galwegian again."