Irish pharmaceutical giant Shire has leased nearly 177,000 square feet at 45-55 Hayden Ave. in Lexington in an expansion of what is already the Boston suburbs’ largest life science campus.

As its Bay State workforce approaches 2,600, the Dublin-based rare disease specialist will occupy two of three former Cubist Pharmaceuticals office and lab buildings next to Shire’s existing Lexington campus, spokeswoman Katie Joyce said.

Shire added 700 employees in Massachusetts last year as part of a corporate restructuring that included the elimination of 600 positions in Chesterbrook, Pennsylvania. It’s reviewing its corporate, R&D and manufacturing in Massachusetts with plans for future growth, Joyce said.

Shire currently has 2,578 employees at facilities in Lexington, Waltham, Cambridge, North Reading, Belmont and Burlington.

Shire already occupies 900,000 square feet in eight buildings at its Lexington campus, which it has designated as its U.S. headquarters. Last year it leased 202,492 square feet at Two Ledgemont Center.

The firm, which developed the attention deficit disorder drug Adderall, is shifting into specialization of rare disease therapies with a series of acquisitions, including its pending $32 billion merger with Bannockburn, Illinois-based Baxalta.

The Hayden Avenue buildings are vacant following the acquisition of Cubist by Merck and the decision to shut down the Lexington research unit in May. Merck bought the 26-acre Hayden Woods campus, including a third building at 65 Hayden Ave., from Cubist in February for $138.2 million. Specifics about which units will be located on Hayden Avenue were not yet available, Joyce said.

Transwestern RBJ represented Shire in the Hayden Avenue lease transaction.

Greater Boston lab vacancies dropped into single digits for the first time in 2015, ending the year at 8 percent, according to Transwestern’s bioSTATus report. In the 2.8-million-square-foot Route 128 west lab market, the availability rate was 5.5 percent at year’s end.