Given that John Garrett (soon to be "engine driver") was in Cardiff on the '41 Census I wondered if any vital events had occurred in the Principality under that name -
Marriage Registration
John Garrett, Dec qtr 1846, Cardiff, 26 614 - on the same page Martha Day, Eliza Thomas, Mary Sullivan, Ann Phillips
Birth Registration
Mary Garrett, Dec qtr 1847, Cardiff, 36 340 - mother's maiden name Thomas
FindMyPast Baptism (image/transcript)
14 Nov 1847, Mary, dau. of John and Eliza Garrett, abode Newtown, place of baptism Saint Mary the Virgin, Cardiff, father's occupation Engine Driver