Amanda:
My McDonnells come from the Co. Mayo/Co.Roscommon border near the town of Ballaghaderreen. My grandfather, Patrick McDonnell emigrated to Philadelphia in 1907. McDonnell is a fairly common name with different "clans" mainly one in Mayo, another in the Northern Ireland and a third in Leinster (Laois, Dublin). Not sure if there is a large number in Limerick.
I'm currently exploring DNA through Family Tree DNA. I've taken the Y-DNA test which strictly focuses on the straight paternal line. (Only males can take the Y-DNA test). There is a Clan Donald DNA project http://www.dna-project.clan-donald-usa.org/
If you have a brother or your father is living you may want to explore Y DNA testing for genealogy purposes. You can take the mtDNA test which looks strictly at the maternal line (your mother, her mother etc) and also an autosomal DNA test which looks at your paternal and maternal DNA and identifies possible genetic cousins. I'm currently waiting for results on the mtDNA and autosomal tests. Very instesting process. I'm just starting to understand a little bit about DNA.
Roger