Maguire enjoyed a successful trial spell at West Ham at the back end of last season and the Premier League club have decided to bring him to Upton Park on a permanent basis.
The 18-year-old has agreed a two-and-a-half year deal at Upton Park with the Hammers having the option of a further year.
Maguire had been attracting the interest of a host of clubs after his displays with Waterford United over the past 12 months and he is regarded as one of the best young prospects to emerge from Ireland in recent years.
The likes of Blackpool, Brighton, Peterborough and Derby had all shown an interest, but West Ham have won the race for the forward's signature.
Maguire, who scored 13 goals in 24 appearances, told West Ham TV: "In summer 2011 the Waterford manager brought me in for a couple of training sessions and gave me a chance in the first team and I took it. Here I am now at West Ham and it's a dream come true really.
"People tend to say my best trait is my movement so hopefully I can show that here. I haven't been told yet which squad I will be in but I presume I'll be going into the reserves so I'll try to work my way up like I did at Waterford."