Rear anti-roll bars have been added to the IRS TR series cars almost since the cars were introduced. Kas Kastner writes up a design in his books from when he was running Triumph's competition department in the US. ADDCO has a rear anti-roll bar kit that is basically the Kastner design. Richard Good at Good Parts has a different design that follows the same principles as the Kastner bar. RATCO and Revington TR out of the UK offer high mount rear anti-roll bars but they are part of rear suspension modifications that relocate the rear damper to run inside the coil spring and the anti-roll bar link then attaches where the damper link did on the tab toward the end of the control arm. Much more work involved including some welding on those two unless you are also starting out with a RATCO replacement frame.
The ADDCO bars should be available from all of the "usual suspects", Roadster Factory, Moss, Victoria British. Richard Good's stuff is available directly from him at goodparts.com or from Roadster Factory. RATCO does stuff through Facebook IIRC. Revington is revingtontr.com out of the UK. Roadster was also selling some of their stuff at one time but can't say if they still do or not.
You might want to consider taking stuff like this to the Triumph section of the board as opposed to the blog. I just happened to run across this by accident. If it had been on the Triumph of the board since the fifth, you'd probably have a page or two full of opinions posted by now.
Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
STOP!! Never post your email address in open forums. Bots can "harvest" your email! If you must share your email use a Private Message or use the 
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
