Tullow Oil plummeted 23c down to €3.78, nearly 6%, following their announcement that drilling on the Faucon-1 exploration well off the coast Mauritania has been completed.
The well will now be plugged and abandoned.
That managed to propel the ISEQ into negative territory, finishing down 37.05 to 7177.28.
The financials enjoyed mixed fortunes today. Anglo Irish Bank closed up 4c to €12.19 while AIB rose 2c to €17.92.
Irish Life and Permanent closed down 19c to €15.94, while Bank of Ireland slipped 10c to €17.92 following the announcement that it has entered into an agreement to acquire 71.5% of Guggenheim Alternative Asset Management, a US hedge-fund manager focusing on institutional and high-net-worth clients, for an estimated $184m (€153m).
Pharmaceutical company Elan was one of the day's heaviest fallers, finishing down 70c to €10.65, a drop of over 6%.
Ryanair fell 6c to €7.75 following the announcement that it has been forced to cancel 600 flights in the New Year due to a delay in the delivery of new planes it has ordered from the aircraft manufacturer Boeing.
The delay has resulted from a strike at the US company during September and October of this year.
The cancellations will affect new routes from East Midlands Airport in Britain and Pisa Airport in Italy during January, February and March.
Ryanair said it expects its schedule to be fully operational by April, when the new planes are due to be delivered.