Brownlee was taking part in his first event at the Ironman distance (3.8km swim, 180km bike, 42km run), and the 31-year-old won the race with ease to qualify for the World Championships at the first attempt.
However, the format was disrupted when the swim leg had to be abandoned off Claycastle beach because of poor weather conditions.
The athletes were left to complete the bike and run legs in driving rain.
Windmill Hill posed another considerable obstacle, topping out with a 21 per cent gradient which caused many competitors to dismount and walk. Nieuwoudt completed the 112-mile ride in 6hr 18min 7sec.
The marathon run was four laps through the historical town, taking in Youghal harbour and the famous Clock Gate Tower, and spectators came out in force despite the downpour.
Nieuwoudt completed the marathon in 4:47.21, finishing the overall race in a time of 11:13.44, 79th in his age group out of 319, while Brownlee won in 7:44.16.
Club mate Nicola Griffiths (F25-29) competed in the Swim-1st sprint triathlon in Crawley.
The event consisted of a 300m pool swim, 19km bike and 5km run and was pitched as ideal for beginners and those wanting a fast time.
Griffiths finished third in her age group in a very strong field, in a time of 1:22.57, putting together three consistent disciplines.
Richard Bysouth(M40-44) took part in Swim the Lake at the stunning Ardingly Reservoir with more than 200 swimmers for company on a midweek evening.
There were three distances on offer, 750m, 1.5km or 3km. Bysouth, competing in the latter, practised his energy-saving tactics and drafted a few fellow competitors until the last lap, where he pushed for a fast finish and came 34th in a time of 52.29.