You can register and search for free on www.scotlandspeople.gov.uk but have to pay extortionate amounts to see the actual images. I found a Terence Tierney born 1904 in the Gorbals (an area of Glasgow). Searching for all Tierneys born 1903-1907 in the Gorbals gives:
Surname Forename Gender Year Ref RD Name
Surname Forename Gender Year Ref RD Name
TIERNEY MARY F 1903 644/12 1419 Gorbals
TIERNEY TERENCE M 1904 644/12 1126 Gorbals
TIERNEY JOSEPH M 1905 644/12 408 Gorbals
TIERNEY ELIZABETH F 1906 644/12 342 Gorbals
TIERNEY CATHERINE F 1906 644/12 1295 Gorbals
TIERNEY STEPHEN M 1906 644/12 639 Gorbals
My guess for 'Sissy' would be Elizabeth.