Hi John, here is an incomplete list of pubs and bars you might want to check out in Calgary. The Folk festival is held on Prince's Island, on the Bow River. Directly south from the Island is Eau Claire, a kind of up-market shopping/condo/restaraunt/hotel area with a number of bars:
Fionn MacCools at the Sheraton Hotel - corporo-celtic, but can host some fine music.
Barley Mill - busy, loud; often live music.
Another bar has replaced the defunct Hard Rock cafe, but I don't know what it is.
A few blocks south you will find Stephen Avenue. Among other establishments, you'll find the James Joyce Pub here. Large, yet cosy; loud, yet intimate. Good sessions, pricey beer.
Also downtown - Ceiligh's- multi-level Irish concept pub. ( It seems you can't get a pub building permit in this town unless it's going to be Irish *BG*)
Cowboys-big beer, big breasts.

North and west of Prince's Island is the Kensington area , where you will find, guess what, more Irish/Celtic pubs. Molly Malone's, Kilberry's. I like the Tullamore, upstairs on Kensington and tenth. One of the first, and a supporter of trad music. (They have a sister pub in Inglewood [9 ave. SE] called Tipperray House.)
Also in Kensington - The Kensington Pub, Bass Brothers, and Original Joe's, which has live music.
There is also an Original Joe's in Marda Loop (33 Ave SW), where you will find the Karma Local Arts house, a great supporter of live acoustic/folk/roots music.

This is by no means an exhaustive survey.
Oh, the King Edward Hotel. Every town in Canada has had a King Edward hotel, methinks. 'Calgary's Home of the Blues'. Just like it says. Visit now before they knock it down to build an underpass.

Hope this helps,
Have a safe journey, and enjoy the festival. Cheers, Bardford

BTW, how much time will you spend before/after the event? There is the South Country Fair in Fort MacLeod the weekend before Calgary. Good lineup. Fort MacLeod is a couple hours south of Calgary.