First, mtDNA tests provide very little genealogical related information but are great for building the tree of womankind. The problem with mtDNA is that it is not a chromosome and is a very small piece of DNA - only 16,000 base pairs in length. This is just not enough DNA to have much of any impact on genealogy. On the other hand, the Big Y is now testing 10,000,000 base pairs out of the 58,000,000. A very new and very expensive test is now yielding 20,000,000 base pairs. So YDNA currently has more than 1,000 times more content that can potentially mutate. So this rules out mtDNA when compared to YDNA.
Second, you next have fork in the road on which test to take next - atDNA or YDNA. You can pursue both as well. There are two huge differences in these tests: 1) atDNA covers all of your ancestors - but is limited to only six generations or less (on the average). This is because atDNA is recombinational where 50 % of the atDNA is randomly passed to each new generation. After only five or six generations, many of your ancestors will no longer be detected since the smaller pieces that are left can not be distinguished from random pieces. So atDNA is great for very recent to back to the 1800 to 1850 time frame. For recent adoptions (the last three or four generations), this is your primary test. The big advantage - it covers all lines - the big disadvantage - it becomes very unreliable in the 1700s. The atDNA test is also much less cost - under $100. However, this is somewhat misleading as many people test their brothers, sisters, parents and cousins.
YDNA is your next choice. This can easily find go back as far as any known genealogical trails back to 1,000 years with no problem. The big disadvantage, any one YDNA tester only reveals information about one of your all male lines. It is also hard to focus on maybe two or three all male lines - each costing the same amount to test. YDNA has by far the most exciting long term potential as academic papers state that every generation will have around five to ten mutations which will allows to easily connect all our lines together - without any names or places but will include approximate dates. For YDNA, I always recommend the FTDNA 67 marker test first. Based on your results, there many other next steps.
I am one of the admins for the second largest Irish haplogroup that exists (R-L226). These are people with Dal gCais ancestry in southern Ireland (related to King Brian Boru). The largest Irish haplogroup is R-M222 is three of four times larger than R-L226 (related to the O'Neil royalty around Dublin). There is extensive testing of these Irish haplogroups (just a fancy name for every that has tested or predicted to be tested for any YSNP branch - such as R-L226 or R-M222). R-L226 currently has 526 testers at the 67 marker resolution or higher. Around two years ago, we could only test R-L226 and did not know of any YSNP branches below R-L226. We now have 47 branches under R-L226 today and adding another branch every two or three weeks. We also have around 130 testers (out of 526) who have tested at least 40 branches robustly.
With all the YSTR information AND YSNP information, we can now chart around 78 % of the testers with an accuracy between 60 to 95 % (this can be verified by just YSNP testing the prediction). This equates to every four of five new testers will immediately know how they are related to all other L226 testers. By the end of this year, we should easily be able to chart 90 % of all 67 marker testers due to many pending tests. 15 branches are genealogical in nature (defined by the fact that more than 60 % share one surname - there will a lot of adoptions over the last 40 generations).
The technology is already available (for L226 at least), we only need more testers with Irish ancestry. For instance, my line is confirmed FGC5639 positive. If you test positive for this YSNP, this means that you belong to Casey line that migrated to South Carolina in the 1740s after the massive crop failures in Ireland during that decade. Since this line is L226 positive, we know that 80 % of these people originated from five Irish counties: Tipperrary, Clare, Cork, Limerick and Kerry.
Here is a chart of L226:
http://www.rcasey.net/DNA/R_L226/Haplotrees/L226_Home.pdfYou really do not have to really understand all the details of this YDNA analysis as there are many very knowledgeable admins to assist you with your results. You do need to set goals and objectives of what your highest priorities are, so that admins can give you advice on what to do next.
If you can prove any of your all male Casey ancestry back to Ireland in the 1850s or earlier and originate in Munster, I will send you a free 37 marker test $139 value (if you will upgrade the test to 67 markers - cost of $99 to upgrade).
