Along with rath and dun, lis means a fort, more specifically the place around a dwelling. As Joyce notes, where rath may have begun as referring to the rampart and lis to the internal space, that distinction faded over time. Lis is common to 1,400 place names (Joyce) but the distribution is mysterious. It displaces rath in the north yet is rare in the east, and along the west coast, notably absent from Donegal. There are heavy clusters in counties Cavan, Monaghan and Roscommon.
Lis has less status that dun (or the rare Daingean, fortress, as in Dingle) or rath and it does not appear in the epic tales, or feature much in the great books of law passed down. It is often combined with a colour – bán, dubh, buí, ruadh, white black, yellow, red – or size, mór or beag, as in Lismore and Lisbeg. Lis can appear at the end of the place name in its genitive form, as in Moyliss (moy, a plain), Knocknaliss (cnoc, a hill), and Gortalasse (gort, a field).
The diminutive Leasán or Lisheen appears in twenty townlands, says Joyce; Lissen Hall near Swords is a good example. Lissadell, famed influenced on W.B. Yeats, means Lios-an-doill, the fort of the blind man (Joyce). The definite article appears too as in Lisnamuck in Derry, the Fort of the Pigs (Lios na Muc).
As Flanagan points out, Lis is often combined with uisce, water; possibly because the ditch outside the rampart was filled with water as an extra defence. Of the twenty-five or so instances of the, most appear only in the west; Lissaniskey for example, Lios an Uisce in counties Cork, Tipperary and Roscommon. If it was a strong fort it might earn the prefix dur, giving us durlas as appears in the anglicized Thurles, Co Tipperary, translated by Flanagan as stronghold.
Lis is at times associated with a name, like Liscolman, or Listowel, Lios-Tuathail, Co Kerry. The Norman Lisrobert, occurs in Co Mayo. Indeed, it appears that in some instances the Norman conquerors took over an existing lis and built their own fortification on top, such the motte-and-baily at Lismahon, Co Down, and the castle at Liscarroll, Co Cork.