Got this via RSS feed earlier this week:
Guitar Hero III®: Legends of Rock will be kicking off the St. Patrick’s Day celebrations with a special free track pack from the popular Celtic punk band Dropkick Murphys.
Straight from the band’s sixth album, The Meanest of Times, the Dropkick Murphys pack will include master recordings of “Famous for Nothing,” “(F)lannigan’s Ball” and “Johnny, I Hardly Knew Ya.” The track pack will be available for free to download on Xbox LIVE® Marketplace.
Guitar Hero III: Legends of Rock downloadable content is instantly playable and available in the Downloaded Songs set list in ALL game modes (Career, Co-Op Career, Quickplay, Multiplayer, Practice and all Online modes). Check out the Marketplace regularly for the latest tracks.
It mentioned that it was free to starting from St. Patrick’s Day, but doesn’t mention any expiration date, so I’m going to assume this isn’t just some limited time promotion. Enjoy!