Celtic *longā 'boat, vessel' is attested in Welsh llong 'ship', Old Breton locou 'vessels, vases', Old Irish long 'vessel, (little) vase, ship', to which corresponds the Gaulish toponymic element Longo-1. There's also Cisalpine Gaulish (Todi) lokan /longan/ 'cinerary urn', an accusative form where /ng/ is rendered as k.
Although some authors have suggested a loanword (with reanalysis) from Latin nauis longa 'warship' (lit. 'long ship'), specialists such as Matasović2 think this is a genuine Celtic word without IE etymology, although I consider it to be cognate to Caucasian *leqˀV 'a k. of vessel' (NCED 1511), where the ejective stop became prenasalized.
Also related is Latin lanx 'dish, plate'3, apprently borrowed from Etruscan in account of its vocalism (lack of distinction a/o).
1 X. Delamarre (2008): Dictionnaire de la langue gauloise, p. 206-207.
2 R. Matasović (2009): Etymological Dictionary of Proto-Celtic, p. 244.
3 This meaning is reflected in Tsezian.