Can I ask, how have you connected Mary Welsh to parents Richard and Ann? Also, from what source do you have father Nicholas showing as born c. 1870 in Ireland?
Have you had any luck on the censuses for the family?
What occupation showed for father Nicholas on Mary Ann's marriage cert?
Any family names for witnesses at the 1921 wedding?
.....Sorry about all the questions

Every piece of info can help sometimes when searches are hard.
Have you gone down the route of searching for a death for mother Mary Welsh. I can see one entry showing for a Mary Welsh/Smith born c. 1873 in 1936 on SP. Not sure if this might be connected (names are not uncommon so hard to know).
Monica
