Yes, traditional Irish stew is just thickened by stewing rough cut potatoes to release some of their starches into the broth. So with mashed potatoes, I think it will be a little bit grittier since the fiber will be incorporated as well as the starch.
It might be a good idea to add spoonfuls of mashed potatoes to the simmering chunked potatoes as they cook until you get the consistency you want.
Would mashed potatoes act also as a sort of thickener? I love the idea of quasi-liquid potato
Yes, traditional Irish stew is just thickened by stewing rough cut potatoes to release some of their starches into the broth. So with mashed potatoes, I think it will be a little bit grittier since the fiber will be incorporated as well as the starch.
It might be a good idea to add spoonfuls of mashed potatoes to the simmering chunked potatoes as they cook until you get the consistency you want.