Polling in most US states takes place between 6am or 7am and 7pm or 8pm US time. This ranges from Eastern Standard Time (EST), which is five hours behind Ireland, through to the west-coast Pacific Time Zone, which is eight hours behind Ireland.
Exit polls should be available for many east-coast states, including swing states such as Ohio, Virginia and Florida, by midnight to 1am Irish time. .
Even if the presidency is not decided until after midnight EST, there will be plenty of clues early in the evening on how things are going.
Mr Obama has more options for piecing together the 270 electoral votes needed for victory, so any early setbacks for Mr Romney could be portents of how the night will end.
At 7pm EST (midnight in Ireland), polls will close in six states, but all eyes will be on Virginia, the first of the battleground states to begin reporting results. If either candidate is comfortably ahead in Virginia, with 13 electoral votes, that could be a leading indicator how the night will paky out.
At 7.30pm EST (12.30am), polls close in three states, including Ohio (18 electoral votes) and competitive North Carolina (15).
By 8pm EST (1am) more pieces of the puzzle will start falling into place as polls close in the district of Columbia and 16 states, including battlegrounds Florida (29) and New Hampshire (four).
Democratic-leaning parts of Florida tend to be the last places to report, so be careful about jumping to a conclusion if Mr Romney looks strong early on. By 8pm EST results will start to roll out quickly. But 4.5 per cent of votes in Florida were not counted on election night in 2008, so if things are tight, no one is going to be hasty about declaring a victo.
At 8.30pm EST (1.30am) polls close in Arkansas (six), where Mr Romney is ahed. By 9pm EST (2am) polls close in 14 states, including battlegrounds Colorado (nine) and Wisconsin (10). Democrats have carried Wisconsin for six straight elections, so a flip here would be noteworthy. – (Agencies/foreign desk)