I can't imagine Anach Cuain ever having been used as a shanty!  These are, as a rule, worksongs used at sea, not songs that just happen to mention it.  As George says,There are unlikely to be any mainstream chanteys in Gaelic, but there was a tradition of Rowing Songs in the Hebrides, which is probably what your friend should be looking at.  Many of these were also used as Waulking songs, and a fairly well-known example which might be suitable is:

(A MHIC IARLA NAM) BRATACH BANA

(The Son of the Earl of the White Banners)

As sung by Calum Johnston of Barra, and recorded by the School of Scottish Studies.

'Ic Iarla nam Bratach Bàna
'Ic Iarla nam Bratach Bàna
'Ic Iarla nam Bratach Bàna
Chunna' mi do long air sàile
(I saw your longship on the sea)
Hi 'illean beag hó ill ó ro
Hi 'illean beag hó ill ó ro
Hù hoireann ó hu ó éileadh

Chunna' mi do long air sàile
Chunna' mi do long air sàile
Chunna' mi do long air sàile
Bha stiùir òir 's dà chrann airgid
(There was a helm of gold on her, and two silver masts)
Hi 'illean beag hó ill ó ro
Hi 'illean beag hó ill ó ro
Hù hoireann ó hu ó éileadh

Subsequent verses follow the same pattern:

Bha stiùir òir 's da chrann airgid
'S cupaill de shìoda na Gaillmhinn
(And shrouds of Galway silk)

'S cupaill de shìoda na Gaillmhinn
Sìoda reamhar ruadh na Spàinne
(Rich red silk from Spain)

Sìoda reamhar ruadh na Spàinne
Cha b'ann an Glaschu a bha e
(It was not in Glasgow that it was [seen])

Cha b'ann an Glaschu a bha e
No 'n Dùn-Bheagan, 's beag o'n làr e
(Nor in Dunvegan, small and lowly)

No 'n Dùn-Bheagan, 's beag o'n làr e
No 'n Dùn-Tuilm nam bratach bàna!
(Nor in Duntulm of the white banners!)

The melody may be found in a number of formats at  JC's Tunefinder.

Malcolm