Hi Hodbarrow,
Welcome. Llanlleiana quarried/worked the same deposit of weathered quartzite/china clay as Porth Wen, which is to the E., to produce bricks, probably something like fire/refractory bricks i.e. high value. It seems that fuel was brought in by sea and the produce exported that same way. The chimney is remote and seemingly unconnected because the flue was built on the ground using the topography to gain flue height; it is also much easier to build like this! The site was destroyed by fire in 1920.
There is some info. on the Penmorfa web site. If you ever find a brick that comes from here, Dave Sallery would much appreciate a photo!
Gwynedd, Inheriting a Revolution. Gwyn. Philimore, 2006.
ISBN 1-86077-432-6 has a few pages on the four Anglesey brick works, with a few attached references. A very good book!
Don't forget the brick works at Cemaes and the Hoffmann kiln. The heritage centre at Cemaes may have some info.; they do have a web site.