Stevie, what is known is that the tender it arrived at Whitehead with is not what it ran with. Rumours abounded for years at Whitehead that it came from a 400, but this is nonsense!
In GSR, let alone CIE days, tenders were swopped. This was common in other countries too. I remember seeing steam locos in Indonesia in the 80s at the end of steam. The particular livery they had involved having the loco number on the loco itself and the tender. So you got B5002 paired with the tender from B5008, B5004 with B5010 and so on.
CIE and GSR were no exception to this. The GNR numbered tenders in a separate series because they knew they'd be swopped. However, swops were generally within a class (but not always). CIE inherited a wide range of non standard locos, so swopping between classes was quite common as well as between locos within a class.
It's probably impossible to say at this stage where 186s tender originated, but we can be sure that it would have travelled around behind many locos of several classes!
Photographic evidence shows that this type of tender was often found with the J15 class. It may well have designed with them in mind, or not!