My favorite cajun food in town is Gene's New Orleans Style on the East side.
1209 East 11th Street
Austin, TX 78702
Great poboys and even better seafood gumbo (served on Friday's and Saturday's only, I think)
Shoal Creek Saloon also has cajun food, but I don't recommend it nearly as much as Gene's. Pretty sure Gene's does not serve boiled crawfish (I think that's what erin s. was looking for?), but Shoal Creek may...in any case Gene's is awesome...good luck.