Tess,
There were 5 Bell farms in Drumbulcan in the tithe applotment records for 1835, including a William Bell. Does that match your understanding of the family? Your website seems to indicate the family were already in Canada by 1831 whereas there is a William Bell still farming in Drumbulcan in 1835. Are you certain the Drumbulcan family is the correct one? (Bear in mind that only a fragment of the 1821 census has survived. Most is lost). The 1901 census for Fermanagh is complete and there were 6 William Bells there then (and 480 across all Ireland), so there are likely to have been similar numbers in 1821 too.
http://www.irishgenealogyhub.com/fermanagh/tithe-applotment-books/parish-of-derryvullan.php
By the time of Griffiths Valuation in 1862 there were 2 in Drumbulcan, James & Johnston Bell.
The birth, marriage and burial records for Derryvullan North parish start in 1803. There’s a copy in PRONI in Belfast and there’s a transcript on-line:
http://www.igp-web.com/IGPArchives/ire/fermanagh/churches/derryvullan-birth.htm
There are very few resources for Fermanagh in the early 1800s and even less for the 1700s and so it will be hard to find much more about the family than you already know.
Co. Fermanagh was heavily settled by Scots from the Border areas, during the period 1610 – 1625. Bell is a common surname in the Borders and so that seems likely (to me) where your family originated. Half the population of Fermanagh has similar origins, so it’s the norm.
The broad historical background to the Reivers arrival in Ireland is that when King James I became King of England in 1603, he was already King of Scotland and so then became the first King of both countries. For hundreds of years the Scottish Borders had been fairly lawless and travellers were routinely robbed, and cattle often stolen and herded across the border by moonlight. The new King was particularly keen to stamp this out because he saw it as an obstacle to commerce between the 2 countries, and being joint ruler that bothered him more than his predecessors.
At the same time he had the problem of Ireland. The Spanish Armada had recently attempted to invade England and further invasions by the Spanish or French were feared. Ireland was seen as a possible jumping off point for such an invasion and understandably, the native Irish could not be relied on to support the English or resist any invasion. So the solution was to plant trusted settlers from England, Wales & Scotland in Ireland, in large numbers, to subdue the native Irish and be on hand to deal with any invasion. King James I was a Scot and so particularly favoured his fellow countrymen. Much of Ulster, including Tyrone & Armagh was heavily settled by Scots. During the 1600s, some 200,000 Scots settled in Ireland representing something like 15% of the entire Scottish nation. They didn’t all come as part of the Plantation. Some settled in the 1640s when General Munro's 10,000 strong Scottish army was disbanded at Carrickfergus after the 1641 uprising, and a further batch came in the 1690s due to famine in Scotland.
As far as the Reivers were concerned, King James I decided to move large numbers of them to Ireland around 1610 onwards. He needed settlers in Ireland and he wanted to get rid of the Reivers from the Borders, or at least stop the criminality by breaking their control of that area. So moving them to Ireland was a bit of a masterstroke which killed two birds with one stone. So if your ancestors are Reivers, they probably arrived in the first 20 years of the 1600s, as part of the Plantation of Ireland.
There are no records of individual settlers at that time. We do know the names of the big landowners (Scots & English) but not of their tenants and others who followed them. DNA might get you back to Scotland but there are no comprehensive paper records in Scotland or Ireland which record individuals names or arrivals.
If you visit Border towns like Selkirk, Hawick, Galashiels & Jedburgh, you will find a strong Reiver tradition with folk & food festivals every summer. You can go for a Reiver walk by moonlight and have a 16th century banquet. The usual nonsense. For example, see:
http://www.hawickreivers.com
http://www.borderreivers.co.uk/Border%20Families/BORDER%20SURNAMES%20%202.htm
If you want a detailed read about the Reivers, a good book is Godfrey Watson’s “The Border Reivers” published in 1974, ISBN 0 709 4478 4. Plenty of bloodthirsty and cruel tales. Bells get regular mention.