Cool, you found the Wrenahan, I've been looking for her. Maybe she raised him after his father died. Will see if I can find them on the 1901 census.
I kind of wonder if maybe Margaret had to send the children to family and friends in order to work. She was a Servant before their marriage, not something you can do easily with five young kids in tow. I found a John Tobin of the right age living with Catherine Tobin in Tipperary or Waterford (can't remember now) in 1901 and 1911 who is listed as his aunt. I can trace a cousin named Catherine of the right age connected to John's father Edmond.
Margaret Ussher Tobin's father was James, so that might be here. Wondered what happened between 1895 and 1921.
Edmond and Margaret Ussher Tobin's children were
1894 Bridget
1892 John?
1891 Margaret
1889 Edmond/Edward died in WW1
1887 Mary
1885 Catherine baptised Bridget, died at 10 months
1886 James died at 1 year
From James to Margaret they lived in Clonmel and all kids were baptised at St Peters and Pauls.
You guys are great, will chase up some of these leads.