From the Official GAA website (http://www.gaa.ie/medical-and-player-welfare/nutrition/hydration/)

Performance may be impaired in some players who are dehydrated before a training session or game, or who drink nothing or little during exercise.

Check out the page on ‘Hydration’ in the Player Welfare Booklet for tips on satisfying your fluid requirements at all times and especially during warm weather.
(Read below for the relevant extract)

HYDRATION
Performance may be impaired in some players who are dehydrated before a training session or game, or who drink nothing or little during exercise.

Fluid Requirements

Prior to Training and Games

• It is important that you are optimally hydrated before training and games
• The urine colour chart (see below)  is a simple tool you can use to assess if you are drinking enough fluids throughout the day to stay hydrated
• Urine should be pale in colour. If it is dark during the day, besides first thing in the morning, fluid intake is required. The target is to achieve numbers 1, 2 or 3. Numbers 4 and 5 suggest dehydration, with colours 6, 7 and 8 indicating severe dehydration

During Training and Games
• If you are optimally hydrated before training or games you should drink if you are thirsty, if not, do not
• There is no need to consume drinks containing carbohydrates during a game

After Training and Games
*Drink approx 1.5 L of fluid for every kg of body weight lost in order to achieve normal hydration within 6 h after exercise

*Commercial carbohydrate drinks are ideal because they provide a source of carbohydrate in addition to water

*Caffeine and alcohol are normally discouraged due to their diuretic effect – increase the rate of urination

Urine Chart