The dean was very pleased with the choir's performance and asked the parishioners for a warm applause, which came promptly. Also afterwards, several choir members were apparently complimented about the singing. 

So Saint Brigid's mass clearly corresponds to Halle's liturgical and popular taste. But how to explain the presence of this Irish composition in St. Gregorius' repertoire? Could it really be down to Seymour's years in Mechelen? Unfortunately, nobody at Halle's oldest choir was able to say how exactly the copy came into their archive.

Of course, Halle is by no means the only place in the world where this score can be found; you can buy a copy at Presto Classical:

http://www.prestoclassical.co.uk/sm/7355699

As it is obviously under copyright protection I won't post any photos of the score here. If you still want to know what it sounds like, Saint Brigid's mass has a rather simple, canon-like structure. Typically the lower voice starts a theme, which is repeated in a modified version by the higher voice. Overall, the composition sounds quite natural and harmonious. 

I seized a rare opportunity to take a look at the cover page of Saint Brigid's mass in what appears to be its original publication (1889), so I'll describe it to you. It is decorated with Brompton-Oratory-style architectural elements, flower motives and human faces. It looks old fashioned, classical, and overly ornamental. The score is dedicated "by permission to the most revered Dr. DONNELLY, Bishop of Canea, honorary president of the Irish Society of St. Cecilia". It also says the mass is composed "for two equal or unequal voices with organ". Finally, it mentions that Joseph Seymour is the organist of St. Andrew's Church, Westland Row, Dublin.

The score is published by CARY & C°, Oxford Circus Avenue, 231 Oxford Street, W. London. The year of publication is not mentioned. At the very bottom it says 'Printed in Saxony'.

P.S. Saint Brigid of Kildare (451-525) is one of Ireland's patron saints, along with ss. Patrick and Columba.