ONlwSG

fleodrainn f. [ˈflɔd̪̥ɾiɲ], gen. idem, ‘buoy, float’ 

Now also ‘a sticky (in computing)’ (AFB^˄).

and fleodradh m. [ˈflɔd̪̥ɾəɣ], gen. fleodraidh [ˈflɔd̪̥ɾi], ‘floating’ have been derived from ON fljóta ‘to run, stream (of water); to float; to be flooded, flow’ (NO).

For fleodradh, see Robertson 1906a, 39; MacBain 1911; and McDonald 2009, 353–54; for fleodrainn, see Robertson; MacBain; MacLennan 1925; Stewart 2004, 410; and McDonald.

The derivation is unlikely: SG fleodrainn and fleodradh have short stressed vowels, not diphthongs as McDonald states, 

The e of fleodradh and fleodrainn is an orthographic device used to indicate that the preceding lateral is a ‘slender’ [l] /l´/ not a ‘broad’ [ɫ̪] /ʟ/.

and would not be expected to result from long ON ó. McDonald (2009, 353–54) suggests that r in the Gaelic words goes back to flýtr, 

Not fljótr as in McDonald (ibid.).

the 3rd sing. present tense of fljóta, and that the ending of fleodrainn may be linked to the past participle flotinn; neither outcome seems probable. It seems more likely that SG fleodrainn (first attested in 1741 

MacDomhnuill 1741, 111: fleodruinn ‘buoy’; Shaw 1780: fleodruin ‘idem’; MacFarlan 1795: fleodrun ‘idem’; MacFarlane 1815: fleodruinn ‘idem’; Armstrong 1825 and HSS 1828: ‘idem’. Armstrong also cites Ir. fleodruinn ‘buoy’, but this is found only in O’Reilly’s (1817; 1864) Irish dictionary, where it has likely been adopted from MacFarlane.

) is either a loan-blend based on Scots flatter ‘to float’ (SND^˄) + a reflex of a Gaelic n-stem ending (and with a ~ o alternation in Gaelic), or a loan from the Scots present participle flatterin ‘floating’.

The ending of fleodrainn also coincides with Gaelic verbal noun endings in -inn: -inn (e.g. faigh ‘to get’ > faighinn), -sinn (creid ‘to believe’ > creidsinn), -t(a)inn (bean ‘to touch’ > beantainn, cinn ‘to grow’ > cinntinn) and -achdainn (fairich ‘to feel’ > faireachdainn).

SG fleodradh, on the other hand, which does not appear to be attested until 1828, 

HSS 1828.

may itself be a derivative of fleodrainn (with substitution of the common Gaelic verbal noun ending -adh), rather than a loan-blend from Scots flatter + SG -adh. For the development of SG fleodrainn from Scots flatter, cf. SG flodraich ‘to lap (e.g. of waves), wash against’ and the verbal nouns flodraich f. and flodranaich f. (e.g. Faclan bhon t-Sluagh^˄), 

The verbal noun SG fleòdradh ‘washing, as of sea washing the shore’ (McDonald 1972: South Uist) appears to be the result of confusion between flodraich etc. and fleodradh, although there must be some other reason for the long stressed vowel, perhaps the influence of Eng. float /fləʊt/ (OED^˄).

loan-blends based on Scots †flotter in the sense ‘to splash, cover with liquid’ (SND^˄); also SG flodraigeadh and (with delenition of f > p via back-formation in Gaelic) plodraigeadh m. in the sense ‘simmering, bubbling’, from the same source but in the sense ‘to bubble in boiling, to simmer, sputter’ (ibid.).

Derivatives: Besides fleodradh, compare the verb fleodraich ‘to float’, the adjective fleodrach ‘floating, buoyant’, the abstract noun fleodrachd f. ‘buoyancy’ and the diminutive form fleodrag f. ‘floater’ (AFB^˄).