This is the famous mother of Richard Neville, the Kingmaker. Political marriages were clearly a powerful social factor at this time:

`... the Nevilles had risen from simple barons to comital rank, ... by useful, though by no means unusual, marriages to heiresses. ...

... Between 1412 and 1436 there occurred what must certainly be the most amazing series of child marriages in English history - eleven marriages involving children under 16 years of age, ... Richard, at 22 or 23 married Alice Montacute, the 15 year old heiress of the earldom of Salisbury. ... By 1434 the children of Richard Neville and Alice Montacute were being put on the market. Young Richard Neville, aged 6, and his sister Cecily, who could have been at most 13, were married to the children of Richard earl of Warwick, Henry and Anne de Beauchamp, aged 8 and 9, as a result of which 15 years later, the younger Richard became earl of Warwick.' (Lander).

Some sources claim Alice Montacute was born 3 May 1415, which would have made her 9 when she was married.

Sources:
Crown and Nobility 1450-1509, J.R.Lander.

http://www.montaguemillennium.com/familyresearch/h_1409_alice.htm