Fifty years ago, they were still going in such numbers to build the post-war motorways and housing estates that they became known as McAlpine's Fusiliers.

Now their prosperous descendants are back in Britain . . . buying up large swathes of the country they helped create. Ambitious and expansionist Irish property speculators and entrepreneurs have moved into Britain in a big way, acquiring major enterprises. According to one report, Ireland has surpassed the US as the single biggest source of outside investment in British commercial property, with more than 20% of overseas purchases last year.

Irish financial investment in Britain is zooming ahead by the billion. And in a new twist, well-known moneymen from the Republic have been joined by lower-profile investors from Northern Ireland.

An Irish spending spree has already netted landmark London hotels such as Claridge's, the Connaughts, and the Berkeley, in addition to large slices of real estate and retail developments.

While estimates vary, it seems likely that investors from the Republic own more than 17bn worth of commercial property in Britain, with some of Ireland's richest men amassing very large investment portfolios. An Irish-owned company is heavily involved in a huge plan to transform Liverpool city centre, while others are pouring money into Luton, Glasgow and Manchester.

At the same time, tens of thousands of cash-rich Irish families are buying second homes or investment properties abroad, in locations as far flung as South America.

Now Northern Irish entrepreneurs are getting in on the act, a dozen or more Belfastbased developers having become substantial players in retail investment, buying shopping centres, retail parks and high street property.

"On behalf of clients, this company has spent about £26bn ( 38bn) in Britain in recent years, " said Keith Shiells, of the Belfast property consultancy BTWShiells.

"There are 12 to 15 heavy hitters, either individuals or partnerships."

One bought a Leicester retail park for £360m ( 520m), while another spent £250m ( 361m) on a larger Doncaster shopping centre . . .

moves known as 'buying chunky' in property jargon.

"Most of them have a good core business, such as house building or licensed trade, and the property has derived from that, " said Shiells. "They're all very low key. They don't have a big corporate profile, they tend to like to keep their head below the parapet."