Kerry is reputed to have 400, and many across Ireland are unrecorded.
Although considered remnants of repressive Catholicism, the origins of cilliní date back to an era when folk tradition and Christianity were entwined.
Now, towns and villages are reclaiming these lonely and forgotten burial places and bringing them back into the community.
Toni Maguire is an archaeologist and anthropologist who specialises in this subject and is best known for her work at Milltown cemetery in Belfast.
"A cillin was any area of ground used for unconsecrated burials, which came under various categories," she says. "This included executed criminals, truce breakers, suicides, mothers who died in childbirth but haven’t been churched, strangers whose religion might not be known, and, by far the largest category, unbaptised babies. There were probably regional differences; for example, in some places it was believed that if a first child died and was buried in a cillin, then the other children would be spared the same fate."
Many academics consider cilliní a post-medieval phenomenon, although an excavation in Galway found graves of infants in a ringed enclosure dating back to 700AD. There was a blurring of the boundaries between Christianity and superstition.
"We often find cilliní associated with fairy trees and, obviously, the strict prohibition against moving such trees would ensure that the graves were not disturbed," Ms Maguire says. "Fairy forts were also used; given that there was such a strong visceral belief in fairies, perhaps they were buried there so that they might have another life with the fairy folk, if denied a Christian afterlife.
"In Orkney, there was a belief that a dragon lived under the fairy mounds, who tormented the souls of the dead. Often, there is this sort of mishmash of folklore, religion and myth associated with these sites."
Ms Maguire says there was trepidation about these unsettled souls. "I refer to them as the dangerous dead, particularly the adult burials," she says. "Boundaries were considered important routes into the underworld and we often find cilliní associated with boundaries, such as running water.
"In Antrim, I found a strong preference for triangular-shaped fields; one explanation is that the three sides of the field represent the Holy Trinity. Another explanation is that ghosts were confined within a triangular field and wouldn’t be able to escape. Again, it seems like an overlay of Christianity, over an older folk tradition."
At the heart of the matter lay the difficult issue of what to do with the pagan dead, within the confines of a rigid and austere Christian doctrine. If a non-baptised child died, then the theory of original sin suggests that this child, although having committed no personal sin, could not enter the kingdom of heaven.
The concept of the innocent children of Christian parents suffering eternal damnation in hell was problematic for the Church; a problem overcome, to a degree, by the theory of limbo.
Probably dating back to the time of St Augustine, limbo was intermediate between heaven and hell; eternal darkness, perhaps, but no pain. It is a theory from which the modern Church has not unequivocally distanced itself.
In a 2007 document from the International Theological Commission, entitled The Hope of Salvation for Infants Who Die without Being Baptised, it is noted that the theory of limbo "remains a possible theological opinion" and that "there is serious and liturgical grounds for hope that unbaptised infants who die will be saved and enjoy the beatific vision. We emphasise that these are reasons for prayerful hope, rather than grounds for sure knowledge."
A distinction can be made between the large, poor grounds and angel plots where non-baptised babies were sometimes buried on an industrial scale, and the small, local plots dotted around the countryside.
"There probably wasn’t a Catholic family in Belfast who weren’t connected with Milltown cemetery in some way over the generations," Ms Maguire says. "A boggy area of Milltown was in use for these so-called pagan burials, up to the 1990s. The daily load of dead babies would arrive from the hospital and were laid in mass graves like carpet."
"One father recalls claiming his dead baby from the hospital and bringing it to the cemetery. The grave digger tossed the dead baby, into what was essentially a wet hole in the ground, like a piece of rubbish. This had a huge emotional impact on parents. We have been working here for years, but it is very difficult to estimate how many babies were placed in the open graves or identify precise locations."
By contrast, burials in the smaller cilliní that pepper the countryside retained a sense of ritual.
"Traditionally, a dead baby would be buried between dusk on the day it died and sun rise the next day," she says. "Often, it was the fathers who buried them, perhaps with the help of someone in the community. In one site, the remains of 21 babies were found in the floor of a roundhouse and there was a line of white quart delimiting each grave. It had been carefully and systematically done, or often simple stones were erected. Apart from community cilliní, there were also personal burial sites. Babies could be buried quietly on a home farm, where two ditches meet. One woman I spoke to couldn’t bear the thought of her dead children being buried in some obscure corner that she wouldn’t be able to visit when she got older. Her husband buried them under the kerb stone at the back door, so that every time she enters the house she walks over them."
While there is still anger about the larger sites, this seems less the case with the small cilliní, which are now being re-assimilated into their communities.
There is a recognition that these were different times and that different rules applied; a sense of compassion and regret rather than anger.
Brian O’Sullivan is part of a committee restoring a small cillin, grown over with furze and bramble, close to the small village of Eyeries, on the Beara peninsula in West Cork.
A similar cillin has been restored in Coulagh in the same parish.
"We were always aware that there was a children’s graveyard here," Mr O’Sullivan says. "It is located close to the church, but it is not associated with it and has no right-of-way leading to it. There are no records for the burials here and it wasn’t until we started clearing the site that we realised how big it was, even though there are people still alive who recall it being used in living memory. It seemed a shame that it had been neglected, and now the ground has been consecrated and the babies buried here have been baptised retrospectively. I suppose things happened in the past that shouldn’t have, but this is really about bringing back a part of the village’s history that should be remembered."