Based at Google's London office, Twohill will report to head of UK operations Kate Burns. She joins from travel website Opodo, which she left in April and where she was head of market development.
At Opodo, Twohill had most recently been responsible for overseeing the entire marketing operation, following the exit of launch marketing director Nicolas de Santis in January.
She joined Opodo from Dreamticket.com, where she was head of marketing, after it crashed in February 2001.
Online travel site Dreamticket.com, which launched in August 2000, was a joint venture between Rapid Travel Solutions and the Ticket Travel Group, but folded due to lack of financial backing.
Prior to Dreamticket, Twohill was with the Irish Tourist Board, where in 1996 she worked on the launch of its award-winning website.
Her arrival at Google coincides with that of commercial director Karen Richardson.
At Google, Twohill will be responsible for marketing its search listings services to businessses and overseeing the launch of new consumer services building on efforts such as its shopping service Froogle.
Earlier this year, Google acquired technology firm Pyra Labs, which is behind the site Blogger.com.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.