The sign with a purple background (left) indicates that the motorist is on Dublin's "outer orbital route" - a route which approximates to the canal ring. Signs for the "inner orbital route" - between St Stephen's Green to the south and Parnell Square to the north - will carry an orange background.
The sign with the white background (far left) indicates the motorist is approaching either of the orbital routes. In this case, the figure 17 on an orange background in the top right corner indicates that the route the motorist is approaching is the inner orbital, and 17 is the number of the junction.
The figure 18 and an arrow indicates the way to the next junction, again as it is in an orange box, a junction on the inner orbital. The P indicates a car park.
On the next level the directional arrow points the way to junction eight, the purple box indicating that it is on the outer orbital. The arrow also points to the regional road R110 and the M50.
On the lowest level, the number 16 on an orange box indicates junction 16 on the inner orbital.
The other signpost on a purple background indicates one is on the outer orbital route. The number six indicates that one is approaching junction six, while the directional arrow points the way to junction five, the national route N11, the M50 motorway and the city centre. The arrow on the second level indicates the way to junction 15 on the inner orbital, depicted by the number 15 on an orange background.