Jane - thanks for 1901 - nowhere near that generation yet !
There is a will of a John Greggor in Episcopal Wills 1841 - the will is dated 1839 and is presumeably that of John II - there are only small bequests but to sons John , George & Benjamin and dau Ann. His executor and residuary legatee is grand daughter Mary Ann Bar - the only other person mentioned is grandson James.
Thus James III was still alive in 1843 - before coming to Ann Bar (or Barr) there is a 1788 property sale from Henry Quayle, mason, of one half a moiety of dwelling house situate in kk Michael st at present in occp Robert Killey, joiner - the buyer, for £26 15 6d was John Griggor, mariner - thus this could be either John I or John II and probably intended for John II who was recently or more likely, just about to be, married.

To return to Mary Ann Barr - she too lives in Peel - in 1841 the obvious Mary Ann age 15 appears to be a servant living away from home - her putative mother is Ann Barr (nee Grigger) married to Irish born James (in 1851 describes himself as musician) - her reported ages in 1841 + 1851 are confusing 34 in 1841, 48 in 1851 and presumeably the same Ann Barr buried aged 61 in 1857 - the most likely explanation (still to be checked) is that the marriage to John Corlett did not last (possibly the older children are Corlett's not Barr's ?)

I still have a couple of unexplained IGI entries
John son Margt grigor 1847 Jan 17 (no husband given - ? is this the widowed Mary Crellin
and
John son John Giggor + mary richardson 1849 May 03

If John III was still alive in 1839 then possibly the William, Jane or Sarah I ascribed to George could be his children