If you have a long board with a straight edge, then you can attach your irregular board to it so that it hangs over the edge of your straight board. Put the straight board's edge against the rip fence on your table saw, and let your board hang over the edge far enough that the blade will cut the edge that you need on your board. After that, put your "helper" straight board away. Flip your board so that the new straight edge is against the fence, and trim the other edge straight.
How can you attach your board to the straight board?
* Clamp it from the top. One drawback is that it leaves screw holes in your "helper" straight board for the clamps.
* Use double stick tape. You risk a dangerous kickback if your board is not securely fastened. It may be hard to separate the boards if you use enough tape.
In case it matters, I use tape and featherboards.