I found a few other things out yhat you may want to use your sources to further investigate. From a Catholic directory of saints I found this information:
St. Farannan died- 590? feastday Feb.15
Abbot and Irish disciple of St. Columbia on Iona,
Scotland. He returned to Ireland to become a hermit at
AllFarannan, now Alternan, Sligo.
St. Farannan - Patron of All-Farannan, now known as
Alternan, County Sligo. Son of Aedh, scion of King Niall.
Went to Iona with St. Comcille to become a monk, was
present at Dromceat convention about the year 580.
Feast day, Feb 15.
Even though the spelling is slightly different, I believe this is
the same as the St. Farnan I've heard about because the
Church in county Kildare says the feast day of St. Farnan
is Feb. 15.
Have I stumbled upon the origination of the Farnan surname? Maybe, if further research points to it. Thanks for your interest