GA /iː/ is a close, front, unrounded vowel. To produce the sound, arch the blade of your tongue as high as you can (without touching) between the alveolar ridge and the hard palate. You should only lower your jaw slightly, so that your teeth are barely parted. Spread your lips apart and pull them back toward the corners of your mouth. /iː/ is phonemically long.
GA /ɪ/ is a half-close, centralized front, unrounded vowel. The tongue is not quite as high as for /iː/ and the lips are more relaxed, still slightly parted. /ɪ/ is a short vowel occurring in both strong and weak syllables. In weak syllables it can be replaced with /ə/, as explained in Chapter 2.
Weak GA /i/ may be somewhat shorter than /iː/, and will be monophthongal in most occurrences, as in easy, radio, Indiana.