The Ireland U19 cap has been on trial with the Hammers since the end of the League of Ireland season and has earned a contract offer from their manager, Sam Allardyce.
Waterford have agreed on a fee with the Irons and Maguire will spend the weekend at the club before having a medical in London on Monday, and once he passes that medical he will be free to sign, on a two-and-a-half-year deal.
Maguire will only be able to officially link up with the Hammers after the transfer window opens next month but he is keen to make an instant impression at Upton Park.
He was top scorer for his hometown club last season, scoring 14 goals in all competitions.
Maguire's Waterford United team-mate Craig Burns also had a trial at the Hammers but Allardyce has opted not to sign the centre half.