1901 Census 3 Raphael Road Douglas RG13/5303 folio 77 page 28:
William Shimmin head Mar 45 General Labourer b Patrick
Margaret wife 43 Loging house keeper b Santan
Margaret dau unm 19 b Peel
Helen B dau unm 17 b Douglas
Leslie son unm 12 b Douglas
Catherine R dau unm 7 b Douglas
Willie BF son 4 b Douglas

Leslie could be Henry Leslie Shimmin on Brian's births index, born August 22 1888 in Douglas to William Shimmin and Margaret Fargher.
Catherine, Helen(a) and William are also on the index.
There are 34 Monas born between 1896 and 1898 on the 1901 census. None are called Mona Irene.
However there is a five month old Leslie buried in Douglas on June 23 1823.
I would hazard a guess that this might be the son of Leslie and Mona, and that his birth certificate would give his parents names.