Thank you both for your replies.
so sorry,Tinker for the confusing title. I was lax with my research which is why I was confused.
My ancestor actually lived in Dowlais not Merthyr ( Merthyr was the district) - I can now see that there was a Dowlais ironworks and a Sirhowy Ironworks. I guess he must have moved between the two during his working life.
The two ironworks appear to be positioned along the heads of the valley road from each other.