SSPX, the Society of St Pius X, was founded in 1970 by a Bishop and group of priests who believed the Catholic Church was becoming too modernist.
In 2012 a much more fundamentalist breakaway faction, SSPX Resistance, was formed.
It does not accept the authority or infallibility of the Pope, nor any of the teachings of Vatican II and the reforms that followed, and it wants to retain the traditional Latin Mass.
The group believed to be operating in Derry is aligned to that splinter group, which is described as ultra conservative and far right in its ideology.
It was founded by a former Catholic Bishop who was twice excommunicated by the Vatican and was found guilty of Holocaust denial in Germany in 2009.
SSPX Resistance Ireland also claims to have held Masses in Belfast, Newry and Cork.
At least one former, defrocked priest is believed to have held masses and administered communion in a community hall the Galliagh area of Co Derry.
Bishop Donal McKeown told RTÉ News that he had heard claims during the past two years that someone who was not an ordained priest was saying Mass in Latin in a community hall.
Earlier this month, he was contacted by a diocese in England that said it had received information that a defrocked priest who was a member of SSPX Resistance Ireland may be ministering in the Co Derry area.
The Bishop wrote to all priests in the diocese informing them that the group was holding Masses and asking them to make all parishioners aware.
The letter was then printed in all Mass bulletins.
"The priests of SSPX Resistance Ireland are not in full communion with the Catholic Church and do not accept the full teaching authority of the Church," it said.
"The priests of SSPX Resistance Ireland administer sacraments, but do so illicitly - that is, without the necessary faculties and approval of the church," it added.
The letter also pointed out the Catholic Church has no supervision of those ministering for the group in terms of safeguarding policies.
It urged "all the faithful to remain steadfast in communion with the church, united with the Holy Father and the bishops who share in full communion with them".
Bishop McKeown said he "wanted to make people aware" that members of the group who are ministering mass "may not have gone through vetting and safeguarding procedures, which is a legal requirement, as well as the fact that they reject everything to do with the Roman church at the present time".
"They would refer to the Mass in English as the Protestant Mass," he said.
"They would reject everything to do with the Church as we have been for this past 50 or 60 years," he added.
He said: "I thought it was responsible for me to inform our priests and ask them to inform parishioners."
Fr Michael Canny, parish priest for the Waterside parish in Co Derry, said it was important to warn parishioners that people may be dressing as priests and purporting to be priests, but are not.
"It is my understanding that there's a small group of people in this area who believe in the teachings of this group and that at present time, there's a concern because a person purporting to be a priest belonging to this group may be ministering in the territory of the Diocese of Derry," he said.
"This breakaway group is not in in line with the church's teaching, or accepts the church's teaching, so they're totally independent of the church's teaching, albeit they sometimes go about behaving just like they are ordained priests," he added.
Fr Canny said the group "have some views that certainly we would not accept", adding "they didn't accept any of the teachings of Vatican II, or any of the teachings of the church since Vatican II and of course they don't accept the promissory of the pontiff".
Promissory of the Pope refers to the doctrine of papal infallibility which states that when speaking in his capacity as head of head of the Catholic Church the Pope cannot error.
The Catholic Church believes this infallibility was a promise given by Jesus to the first Pope, Peter, and passed down to his successor.
Fr Canny said that while the number of people in Co Derry believed to be attending Mass ministered by SSPX Resistance Ireland is small, the church was duty bound to inform them.
"Should something happen with regard to safeguarding with regard to this group, then the people certainly have been warned," he added.