Ir names where you go, so it wants a destination. Irse names the leaving itself: you are off, and where you go next does not matter.
Where the choice changes the meaning
| Context | ir | irse |
|---|---|---|
| Destination | Voy al mercado — names the place you go to. | Me voy al mercado — you leave for it, and the leaving is the point. |
| Leaving on its own | Voy alone is incomplete. | Me voy — I am off. No destination needed. |
| Commands | ¡Vamos! — let's go. | ¡Vete! — go away. |
| Set phrases | Ir de compras, ir de vacaciones. | Irse de la fiesta — to leave the party. |
| The near future | Voy a comer — ir a plus an infinitive. | Not used for the future. |
| With a gerund | Va mejorando — it gets better bit by bit. | Not used. |