August 25 is the commemoration of a saintly abbot of Moville, although whether this was the County Down monastery founded by Saint Finnian or another foundation in the County Donegal location of the same name is open to debate. Although not a great deal is known of Saint Sillan as an individual, his feast is well-attested on the Irish Calendars, as Canon O'Hanlon records:
St. Sillan, Bishop and Abbot of Magh-bile, or Moville. [Sixth and Seventh Centuries.]
In the published Martyrology of Tallagh, a notice of this holy prelate's parentage and place of residence will be found. At the viii. of the Kalends of September, or the 25th of August, his name is likewise to be met in the Tallagh Martyrology contained in the Book of Leinster; and there, besides his being called Bishop and Abbot of Magh-Bile, he is said to have been son to Findchain... The present holy man must have been born about the middle of the sixth century. Where his birth took place does not seem to be known, but probably it was in the northern part of Ireland, and most likely he was trained in a school established in Moville, County of Donegal, at a very early date. However, others consider him to have been connected with Magh-bile, or Moville, in the County of Down.
...It would seem that St. Sillan flourished towards the close of the sixth and the beginning of the seventh century. About this time, he lived in or presided over the ancient monastery founded at Moville. Tighernach and the Annals of Ulster simply style him abbot. The Annals of the Four Masters and of Inisfallen, at 613, call him both bishop and abbot; but the territory in which he actually lived has not been indicated. In our Calendars, Martyrologies, and Annals, Maghbile is often mentioned, and in a general and an absolute manner, without any allusion to a second monastery of that name.
...At the year 613, the Annals of Innisfallen call this saint, a bishop and an abbot, when recording his death. This event is placed at 618, by the O'Clerys, both in the Martyrology of Donegal, and in the Annals of the Four Masters. According to the statements of Tigernach, the Chronicon Scotorum, and the Annals of Ulster, his demise is recorded at 619. Under the head of Magh-Bile, at this date, Duald Mac Firbis enters Siollan, who is called the son of Fionchan. He is, likewise, styled bishop and abbot of Magh, or Maigh-Bile. Without particularizing his locality, at this same date, the Martyrology of Donegal designates him, Siollan, Bishop and Abbot of Magh-bile. The Irish Calendar, belonging to the Irish Ordnance Survey Records and that at present preserved in the Royal Irish Academy, has a notice of this saint, at the 25th of August.