How to Use DNA for Irish Genealogy: The Leeds Method Guide

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The Truth Teller

Paper records can lie. People can lie about their age, their name, or who the father really was. DNA does not lie.

When the paper trail runs cold in 1840, biology takes over. This week, we aren’t looking at old books. We are looking at the bloodline itself. It is the final, undeniable tool to break down the walls that the 1922 fire created.

The Problem: Too Many Matches

If you have taken an Ancestry or 23andMe test, you probably have thousands of “4th Cousins.” It is overwhelming. You can’t research them all. Most people stare at the list, get confused, and close the tab.

The Solution: You need to sort the chaos. You need the Leeds Method.

What is the Leeds Method?

Created by Dana Leeds, this is a color-coding system that sorts your DNA matches into four distinct groups.

  • The Goal: To identify which of your four grandparents each match is related to.
  • The Result: Instead of a list of 5,000 strangers, you have four clear teams: The “Murphy” team, the “Kelly” team, the “Ryan” team, and the “O’Shea” team.

Step 1: The “Sweet Spot” (90cM – 400cM)

Ignore the really close matches (siblings/aunts) and ignore the distant ones (15cM). Filter your matches to show only people who share between 90 cM and 400 cM with you.

  • Why? These are usually 2nd or 3rd cousins. They are close enough to be significant, but distant enough to separate the branches.

Step 2: Assign the Colors

  1. Open a spreadsheet (Excel or Google Sheets).
  2. List the first match in the “Sweet Spot.” Assign them the color Blue.
  3. Look at their “Shared Matches.” Everyone who matches them also gets the color Blue.
  4. Move to the next person on the list who doesn’t have a color yet. Assign them Red.
  5. Color all their shared matches Red.
  6. Repeat until you have 4 distinct color groups.

Step 3: Identify the Branch

Now look at the trees of the people in the Blue Group. If three of them have “O’Brien from Clare” in their tree, you know the Blue Group represents your O’Brien grandparent.

  • The Breakthrough: If you have a “Mystery Match” in the Blue Group with no tree, you still know they must be an O’Brien. You have solved the mystery without a single document.

🤖 AI Shortcut: The “Shared cM” Calculator

Confused by the numbers? Use AI to predict the relationship.

The Prompt: “I share 250 cM with a DNA match. They are in my maternal grandfather’s cluster. What are the statistically most likely relationships between us? Exclude relationships that would require them to be under 20 years old.”

Congratulations! You have completed the course.

You now have the roadmap. The records are waiting.

Want to keep this guide forever? This is the final lesson of our free online series. But you don’t have to search through 12 different blog posts every time you need to check a fact.

[Download the entire 12-Week Masterclass Guidebook as a PDF] (Contains all 12 lessons, advanced checklists, and printable worksheets)

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