The expansion would be a major boost to Ireland's international profile as a centre for technology research and development.

The 1,400 employee milestone puts Google five months ahead of the recruitment target it announced last November. Google had just five employees in Ireland four years ago.

The additional staff would require Google to acquire a third building, preferably close to the two Google currently leases at Barrow Street near Grand Canal Dock.

Last week Nelson Mattos, Google's new head of engineering for Europe, said he intended to hire thousands of engineers for research and development work to be based in Google's European offices.

"I aim to grow the EMEA [Europe, Middle East & Africa] engineering team as big as the one in North America. That is why I joined, " he told the Financial Times.

Dublin is already the second-biggest Google office after its headquarters at Mountain View, California. Google employs 13,786 people worldwide and would see that total grow by a third, mostly in Europe, according to Mattos.

Informed sources say this could see Google "at least double" its Dublin headcount in 12 to 18 months.

The main limit to the numbers employed could be space.

Under its current lease arrangements, Google has space for roughly 400 more staff, it is understood. If its rate of recruitment were to stay at 20 per week it will run out of space in less than six months. Google has said it needs to be in a city centre location.

A Google spokesman said Mattos was travelling and unavailable for comment late last week.

While the majority of Google's 1,400 Dublin staff are understood to be involved in administration, sales and marketing of online advertising, the number of engineers has already begun to rise and may already be greater than 150. To date the engineering work in Dublin has been limited to keeping Google's main search website operating well. But lately the company's Irish jobs website has listed dozens of open positions for application developers.

It has also, to little notice, substantially increased the amount of location information on the Irish portion of the Google Maps website.

Much of the growth in Europe is to "localise" its products to make them seem less American in local markets.