Just to get the ball rolling on this area here are a few links people might find interesting regarding Medieval Ireland.
The Hobelar
http://www.scottishhistory.com/articles/independence/_derived/sourcecontrol_hobelar.htm
Medieval Irish Plea Rolls (PDF)
http://www.nationalarchives.ie/topics/Medieval_plea_rolls/Medieval_Irish_Plea_Rolls.pdf
The Medieval town fortifications of Limerick
http://www.ijai.supanet.com/vol1/kw/limerick.htm
The campaign against the Scots in Munster, 1317 (PDF)
http://www.deremilitari.org/resources/pdfs/frame.pdf
King John's expedition to Ireland, 1210: the evidence reconsidered
http://www.deremilitari.org/resources/articles/duffy.htm
Military Archery in Medieval ireland: Archaeology and History (PDF)
http://www.deremilitari.org/resources/pdfs/halpin.pdf
Irish Frontier Warfare - A fifteenth century case study (PDF)
http://www.deremilitari.org/resources/pdfs/ocleirigh.pdf
Warfare in the Medieval Gaelic Lordships (PDF)
http://www.deremilitari.org/resources/pdfs/simms.pdf
There are a few books you could try as well:
Galloglas: Hebridean and West Highland Mercenary Kindreds in Medieval Ireland
http://www.amazon.com/Galloglas-Hebridean-Highland-Mercenary-Kindreds/dp/1862322511/ref=pd_bbs_sr_6?ie=UTF8&s=books&qid=1208767197&sr=8-6
The World of the Galloglass
http://www.fourcourtspress.ie/product.php?intProductID=484
There are a lot more out there so please add to this list and we can get a cracking reference list.