[Buzz]

Not much to go by from the pics, except that it does indeed have a Jag IRS, but the key takeaway is that it has 4" round main frame rails.
Of the major manufacturers in the 90's that used Jaguar rear suspensions, four come to mind: ERA, Contemporary, Unique and Everett Morrison - all well-respected and generally highly regarded brands. Of those four, only Contemporary and Everett Morrison featured round tube main frame rails, so my initial guess is that it is one of those two.
They both had similar body shapes, and its quite likely that the Everett Morrison body was a splash of the Contemporary. If I had to guess between the two, I would say its likely to be Everett Morrison, as Contemporary is/was (debatably) regarded as the premier 'glass bodied replica in its day and most owners are very conscious of the brand. If it is a Contemporary with a well known signature CCX---- vin number, in all likelihood it would be openly advertised as such.
Having said all of that, without better photos of the body, interior, engine bay, chassis and suspension; this is pure speculation and it could be something else entirely.