The Leahy / Lahey / Lahy families of County Cavan have a long history in the County. They are a distinct group of 'Leahys' isolated from the other concentrations of Leahyfamilies in Ireland. Several theories of their origins are explored along with rationales and what little evidence that exists to support them. After many years researching the Leahys in Cavan I have come to the conclusion that they (and Leahysin neighbouring counties) are all related. I have studied Church and Civil records along with census data, Wills and most importantly Land Deeds records as well as someprivate collections and have been able to create a 'Top of the Tree' structure thatfits in with all the known data. There are still plenty of holes and isolated groupsthat can't yet be connected, however I present what I see to be the most accurate Leahy family tree in County Cavan from the late 1600s to about the year 1800. This volume is about the 'Origins' and the most early Leahy / Lahy individuals that are on record. Hopefully it might provide some insight to the Leahy descendants worldwide who can't get much further back in their family history research than 'Cavan'.A second volume will describe the various Leahy groups from 1800 onwards