IRL = In Real Life - refers to the real world / outside the game environment (which may in some cases extend to another form of electronic communication).
It is typically used to express relationships between players (e.g. "I know X IRL"). I can also be used to delimit in-game statements from OOC statements - for example, to express excuses for inactivity or other moods during the game (to distinguish from in-game role hints) e.g. "I was busy all night IRL"