Shelbourne Development is understood to have agreed terms with Deka, the German investment fund that paid £186m for the building in 1996, people close to the company said.
Deka, which is being advised by FPD Savills and CB Richard Ellis, put the property up for sale in November. Shelbourne, led and majority owned by Garrett Kelleher, a 42-year old Dubliner, is understood to have been one of seven bidders, including Buckingham Securities.
The transaction is one of the largest by an Irish company in the UK market. It follows a trend of recent purchases by Irish developers, who have been looking to the UK and London in particular as their home market has cooled.
The last big Irish deal in London was the £246m purchase in 2001 of the former Daily Express building by Green Property and a group of private Irish investors. The Fleet Street building is now the headquarters of Goldman Sachs.
Shelbourne is expected to fund the Lloyd's deal with 80 per cent bank debt. The property is currently let to the Society of Lloyd's under a leaseback arrangement, for a term expiring in February 2031.
The building was purpose-built as an insurance marketplace for underwriters and Names - the wealthy individuals who provide capital to underwrite risk in Lloyd's - by Richard Rogers, the award-winning architect.
In addition to the main market floor, it has more than 348,000 sq ft of office accommodation, generating rental income of £16.8m a year.
This implies a yield of about 6.5 per cent, which compares favourably with other recent transactions in the City.
Mr Kelleher, a schoolboy tennis star, first became involved in the property market doing industrial loft conversions for office and retail in Chicago in the late 1980s and early 1990s.
Mr Kelleher's partner is Chris O'Connell, who formerly worked as a corporate finance director at the Bank of Ireland and before that with Kleinwort Benson in London and Frankfurt.