Meer over de gang naar de stembus in Ierland over het nieuwe EU-verdrag (en)
The polls have opened in Ireland to allow citizens to vote on the EU treaty with Irish prime minister Brian Cowen qualifying the referendum as "an important date in Irish history."
Some three million citizens are eligible to cast their ballot - polling stations are open from 8am to 11pm CET.
They will be asked whether or not they approve an amendment to the constitution to allow Ireland to ratify the Lisbon Treaty.
With turnout seen as crucial to the eventual outcome - a low voter show of around 30 percent is likely to result in a rejection of the treaty, a higher turnout of well over 40 percent is more likely to result in a Yes - the weather is also set to play a role.
The forecast would appear to favour the yes camp with the Irish weather station predicting a dry day with just a "few scattered showers."
The first exit polls are expected after 11pm while counting will begin on Friday morning. Results are expected early on Friday afternoon.
Final opinion polls showed the two sides almost neck and neck. An Irish Times poll at the end of last week put the no side in the lead by five percentage points, a Sunday Business Post poll a few days later put the Yes side in the lead, but only just (42% to 39%).
Mainstream political parties in Ireland all back the treaty. Mr Cowen has been appealing to Irish citizens' generally pro-European outlook ahead of the vote suggesting that a rejection would have negative political consequences for the country.
"After 35 years of respecting and helping Ireland, the EU has earned the right not to be seen as a threat to us and I believe that the Irish people will reflect on this when examining the arguments of both sides of this campaign," he said on Wednesday.
The No side, which says the treaty will undermine the country's sovereignty and threaten its stance on neutrality and abortion, has been arguing that a rejection of the treaty will mean a better one will be negotiated.
Ireland is the only country to have a referendum on the treaty and is being closely watched around Europe. In 2001 the country rejected the Nice Treaty before voting in favour of it the following year.
With the no side gaining ground in the last weeks, politicians from larger member states have criticised Irish citizens for appearing to be ungrateful for all the EU aid it has received over the years.
A headline in today's FT Deutschland states: "Europe feeds but still the Irish could bite"