Gap Year is taken between 2nd and 3rd level education at the persons own impetus - generally used to backpack around SEA and/or work for a bit in the 'real world'.
Transition Year is the 4th year of 2nd Level in Ireland - the conclusion of the 'Junior Cycle' of secondary school, culminating with the 10-14 Subject 'Junior Certificate' exam at the end of 3rd year. It is a regular year in terms of school attendance, but rather than academic modules structure against a strict curriculum, students tend to do a mix of practical and theoretical skills.
Things like intro to psychology, philosophy, an esoteric language or other college accession style intro classes are popular. Photography/Film Making, Cooking, Self-Defense/Martial Arts classes are all popular on the practical side.
Most of the religious schools (i.e. about 80% of all schools) tend to have a charity component as well - seeing TY students out collecting for Irish Cancer Society or packing bags in a supermarket to fund a local Hospice is pretty standard.