Dublin and Wicklow, or the whole west coast: which pack
Four days in Dublin, with a train out to Howth or a coach to Glendalough and the Wicklow Mountains, fit in the 3 GB pack, valid for 15 days. Flights from North America also land at Shannon, and from there most people work along the west: the Cliffs of Moher, the Burren, Galway and Connemara in a week of driving is a 5 GB trip, most of it maps, B&B bookings and photos.
The longer loops are where the bigger sizes pay off. Add the Ring of Kerry and the Dingle peninsula, or follow the Wild Atlantic Way up to Donegal over two weeks, and 10 GB leaves room for a passenger navigating on a tablet from your hotspot. The 5, 10 and 20 GB packs all run for 30 days, so the 20 GB one suits a month working remotely from Dublin or Galway.