Annalise Murphy remains at the top of the Laser Radial fleet having placed 8th and 19th today.
Ryan Seaton and Matt McGovern slipped to 9th overall in the 49er fleet and James Espey is 45th overall in the Laser class.
The Star class were on a rest day and remain ninth overall whilst the last of the Irish sailors, Ger Owens and Scott Flanigan, will begin racing in the 470 class tomorrow.
By Niall O'Flynn in Weymouth
Past the half-way point in the Olympic regatta, and Ireland's Annalise Murphy is still holding on to gold medal position - despite a tricky day on the Weymouth seas.
After four wins in four days in the Laser Radial class, the Dubliner finished down the field in today's two races, passing the finish line in eighth position in Race 5 and 19th in Race 6.
The fleet has a day off tomorrow, and the 22-year-old student will take a small lead into Race 7 of the 10-race series.
Whatever happens, she is almost certain now to compete in the Medal Race, Race 11, where the top ten boats will race for Olympic glory.
Her nearest rivals, however, have dramatically cut the gap to the Irish woman. Belgium's Evi Van Acker is in silver medal position, with 14 points - just two behind Murphy, whose performance today moved her from a perfect four-point score to 12. Marit Bouwmeester, of the Nederlands, is in bronze position, with 19 points.
"It was a very tricky day today, but the first race I am pretty happy with - it was a solid race. The second race, 19th is not the worst result in the world... I think I managed to turn a disaster into something less than a disaster," said Murphy afterwards.
The Rathgar sailor, who has dominated the first half of this Olympic campaign, blamed the "shifty" winds for her less than perfect day. Winds of up to 30 knots had been forecast for today, conditions which would have been ideal for the tall, strong Murphy. But those winds never materialised, and Murphy found herself fighting to get clear wind.
"Everyone is having up and down races. The fact that I had such a good first two days has given me a great start on everyone else. It's good, over half-way through a regatta to be in first place - but there's a long way to go."
In the 49er category, it was a difficult day, too, for Ryan Seaton and Matt McGovern, who have slipped from 6th overall to 9th. They finished 12th and 19th in their two races today.
Seaton and McGovern had a similarly challenging day in the 49er class. They rounded the first mark of Race 5 in 15th place, but thanks to some tactical sailing they overtook three of their competitors to finish the race in 12th.
Unfortunately Race 6 proved to be their most difficult to date and they finished towards the back of the fleet in 19th.
The 49er discard also came in to play today so the pair discount the 19th and now sit 9th overall on the leaderboard on 41 points.
The leaders Nathan Outteridge and Iain Jensen of Australia are on ten points followed by Peter Burling and Blair Tuke of New Zealand on 23 points and Emmanuel Dyen and Stephane Christidis of France on 30 points.
The 49er class will continue to race tomorrow and still have a further nine races to sail before the medal race.
“The boys aren’t even half way through the fleet races and are still within the top ten. Everything is still up for grabs” remarked O’Callaghan.
James Espey also had his third day of racing today in the Laser class.
The Laser fleet also had to contend with undoubtedly frustrating postponements and recalls. Espey finished 47th in Race 5 and an improved 42nd in race 6.
With his discard he lies 45th overall. The Laser class will also have a rest day tomorrow with racing resuming on Friday.
Today was the rest day for Peter O’Leary and David Burrows and the rest of the Star class. The Irish pair remain in 9th overall and will resume racing tomorrow.
Racing will continue tomorrow for the Star and 49er.
Ger Owens and Scott Flanigan will also make their debut in the 470 class. The Laser Radial and Laser are on a rest day.
Sailing action starts at 12 noon in Weymouth.