Saint Riaghail of Muicinis, October 16

The Irish Calendars commemorate Saint Riaghail of Muicinis on October 16, a saint who may also have been honoured in Scotland. There he was later confounded with an eastern saint said to have brought the relics of the apostle Andrew to Scotland. The Monasticon Hibernicum has this to say of Saint Riaghail and his locality:

Mucinis, Regulus, who was living in the time of the great St. Columb, was abbot of Mucinis, in Lough-Derg, bordering the County of Galway; his festival is held here on 16th of October.

to which Bishop Moran has added:

The name of St. Regulus, in Irish Riaghail, is entered in the Martyrology of Donegal, on the 16th of October: — “Riaghail, Abbot of Muicinis, in Loch Deirgdheire." He is also commemorated in the Felire of Aengus with the strophe: — “Riaghail, gifted was his career." His Acts are inserted in the Breviary of Aberdeen and he is especially honoured in St Andrew's, in Scotland, on March 30th and October the 17th.

Rt. Rev. P.F. Moran, ed.,M.Archdall, Monasticon Hibernicum, Volume II, (Dublin, 1876), 223.

I was left unsure if our Irish saint is the same as the saint venerated in Scotland and turned to Bishop Forbes' work on the Scottish calendars to see what evidence he could muster:

REGULUS or RULE. March 30 and October 17.—The commemoration of the Scotch S. Regulus occurs in the Breviary of Aberdeen on March 30, the same day as that of "S. Regulus or Rieul, who, having converted the country of Senlis to the faith, about the same time that S. Dionysius preached in France, was made first bishop of Senlis, and died in peace in the midst of his flock."—(Alban Butler ad diem, on the authority of the Bollandists and Tillemont.) So Usuardus, "Apud castrum Silvanectensium, depositio Sancti Reguli episcopi et confessoris."—(Ed. Seller, p. 180.) Another day, the 17th of October, is also kept in his honour: Quoniam in quadragesima de eo non fuerit servitium, omnia sicut in alio festo, sed differtur in crastinum (Brev. Aberd. pars estiv. fol. cxxviii.); and it is remarkable that the 16th of October is the day of S. Riaghail, abbot of Muicinis, in Lough Derg on the Shannon (Mart. Donegal). He is commemorated in the Felire of Aengus at October 16th as

Riaguil raith arremsin.
[Riagail gifted was his career.]

which is glossed by
i. Riagail Muicindsi fa Loch derc.
[i.e. Riaghail of Muicinis in Loch Derg.]

The Breviary of Aberdeen says that he is specially honoured in S. Andrews and its diocese. The Martyrology of Aberdeen associates him with the church of Kylrewni...

Alexander Penrose Forbes, D.C.L. Bishop of Brechin, Kalendars of Scottish Saints, (1872), 436-437.

Thus it would appear that it is the close coincidence of the feast days of the Irish saint and of the Scot  which suggests they are the same person. The Irish Bishop Moran, however, presents a plausible case for the presence of our Saint Riaghail in Scotland in another of his works. After establishing that the original name for St Andrew's was Kilrigmond, he says:

The first mention which we find of Kilrigmond points it out as a monastery of Irish religious, and, among other saints, the patron of Kilkenny, St. Canice, pursued there a life of holy seclusion for some  time. Even St. Regulus himself is found to have been one of those Irish saints, and his name is none other than the Irish Riaghail. He was a contemporary of St. Canice, and famed in our early church as abbot of Muicinish, in Lough Derg, on the Shannon, and, like many of our saints, it is probable that he made North Britain the theatre of his missionary zeal, and closed his days at Kil-Rigmond. It was only in the year 736 that the Pictish monarch, in gratitude to God for a great victory which he had achieved, erected there a church in honour of his patron, St. Andrew, which he enriched with vast possessions, and which in the course of time became the royal and primatial church of the whole kingdom. At the time that St. Andrew's was thus founded, Kil-Rigmond had the Irishman, Tuathal, for its abbot, whose demise is recorded in the Annals of Tighernach under the year 747.

Right Rev. P.F. Moran, Irish Saints in Great Britain (Dublin, 1879), 200.

Note: For more on the legend of Saint Regulus see here.