Catherine Disney (1800-1853) is known as the ‘lost love’ of the Irish mathematician Sir William Rowan Hamilton (1805-1865), but about herself little is known. Based on what Hamilton wrote about her and scraps of information which were found on various places, extended with conclusions which could be drawn from known events, this is a sketch of how she fell in love with Hamilton in 1824, what the motives may have been for her family to force her to marry the reverend William Barlow (1792-1871), what may have triggered her suicide attempt in 1848 after which she did not live with Barlow any more, and how she spoke with Hamilton shortly before she died. In these two interviews she could finally tell Hamilton that she had also loved him. In the last chapters it is discussed how Catherine’s unhappiness seems to have influenced her eldest son, James Barlow (1826-1913), and through him also her granddaughter Jane Barlow (1856-1917). This sketch is supplementary to the essay A Victorian Marriage : Sir William Rowan Hamilton. But being self-contained, it can also be read on its own.