We've had Celtic music; we've had Celtic mythology; we've had, God help us, Celtic spirituality. You might imagine that certain areas of human endeavour would be beyond the reach of the current craze for all things Celtic - the crime novel, for instance. But you'd be wrong. Sister Fidelma, a nun who charges happily around seventh-century Ireland solving murder mysteries, is given her 11th outing in Peter Tremayne's latest book, Smoke in the Wind. Miss Marple in a wimple? Not quite. Fidelma is straight out of the sleuth noir mould: dark, handsome, a qualified lawyer, an expert horsewoman and - rumour has it - not entirely averse to the idea of sharing a sleeping bag with her sidekick, Brother Eadulf.
However outlandish this may sound, her creator insists that he didn't make it up. "Everything that happens in the Sister Fidelma books is absolutely accurate, historically," says Peter Tremayne, aka the Celtic scholar Peter Berresford Ellis. "Under the Brehon laws, women could aspire to be the equals of men in all the professions. They could be doctors, lawyers, teachers, clan leaders, judges." There was even a female bishop, Brigid of Kildare, who died in AD 650.
"Women could also get divorced as easily as men. There were nine reasons for divorce, one of which was if your partner snored. And of course there was no hang-up about celibacy. It was only after the eighth century that all that began to erode, as the Celts moved more into the Roman idea of legal concepts; but even in the Roman church celibacy wasn't enforced until the time of Leo IX in the 11th century. Then it was enforced very brutally. Loads of the wives of priests committed suicide because the good Christian pope ordered them to be rounded up and sold into slavery. Great stuff, wasn't it?"
Great source material, more like - and Peter Tremayne isn't the only one to have mined it. Inevitably, Sister Fidelma has been compared to Ellis Peters's Brother Cadfael, the mystery-solving monk. "Critics have called her Cadfael's successor, which is nice - but only in the literary sense, because Fidelma is actually eight centuries earlier." Even in the literary sense, however, a race between the two spiritual sleuths is out of the question. "Ellis Peters died a few years ago. She was in her 60s when she got hold of the Cadfael character. It's very sad, really. The success came too late for her to enjoy it."
But Tremayne, who effortlessly brings forth information on the nitty-gritty of daily life in Celtic Ireland, is noticeably reticent when it comes to giving away any secrets about his own heroine's literary future. "She has what we call sexual tension with Eadulf," he says, as if that explained everything. "And there have been, um, developments. I do, though, get quite a number of letters from priests and nuns who would - I suspect - rather like to hark back to those days . . ."
In his "real" incarnation as Celtic Studies scholar, Peter Berresford Ellis is the author of a fistful of non-fiction books including The Celtic Revolution, The Celtic Empire, A Dictionary of Irish Mythology, Wales: A Nation Again, Caesar's Invasion of Britain and The Problem of Language Revival.
He is also the author of the definitive history of the Cornish language and literature: "Definitive because it's the only history," he says ruefully. His mission has always been to inform the wider public about his chosen fields of interest - so even his Fidelma novels contain explanatory prefaces. But, he says, there's a huge amount of information out there which has - due to the lack of research funding - yet to be processed.
"During the Tudor conquest, a lot of the old Celtic chiefs fled, and they took their bards with them. So all over Europe - in Vienna, for example - there are huge numbers of documents that haven't even been catalogued, let alone translated. The same thing happened during the early missionary period, when missionaries were setting up places like Regensburg. What we know of Irish mythology is founded on about one-tenth of known manuscript sources - nobody has ever bothered with the rest. I don't want to launch an attack on academia; it's just that the academics tend to bang on about the known stuff, when what's needed is to get into Europe, find those manuscripts, and even just catalogue them, to start with." Still, it may come as something of a surprise to readers accustomed to the Roman view of history (Celts = smelly savages) to discover that the seventh-century Irish were not only assiduous about personal hygiene, but also keen aromatherapists, bathing each evening in tubs that contained sweet-smelling herbs, then towelling off with linen cloths.
Or that the reason why we contemporary Celts often pronounce the word "film" as "fillum" is due to a transference of Irish pronunciation rules into English. Small wonder that Sister Fidelma has her own website, set up by a devotee in Arkansas, which features historical notes, pronunciation guides and such-like (www.sisterfidelma.com).
Celtic chic may have spawned everything from Riverdance to born-again Druid separatists in Derbyshire, but the more we learn about the realities of Celtic society, says Peter Tremayne, the more we'll learn about ourselves.
Smoke in the Wind by Peter Tremayne is published by Headline at £17.99 in UK