Catharsis is in Gm, so appropriate combinations of Gm F Ebm and D generally work. A basic pattern to start off with goes like:
Gm /Gm /Gm /D /Gm /Gm /Ebm /D
Gm /F   /Ebm /D /Gm /F   /Ebm /D

When you get to the end of the phrase you can run back up from the D back to the Gm; D Ebm/F G , just to add a bit of interest.   

For the version of Tam Lin that I know (Am) the chords go something like this:

Am /F /G /Em /Am /F /G /Em Am
Am /Am /G /G /Am /Am /F /Em Am

Transpose them all down a 5th to get the Dm chords.

You can substitute the second lot of Ams in the second part with Fs the last time through for variety. Or alternatively, sit around and play the tune lots of times until you get bored and start mucking around with the chords to make it more interesting, that's usually a good way of inventing variations!

Hope this helps

Charley