O'Shea/Shea and Shannon

[ELMurphy]

Hi, I have a Denis/Dennis (b abt 1836) and wife Bridget (b abt 1833) )O'Shea/Shea in St Pancras, London in the 1861 census. Denis has a sister Margaret (b abt 1833) and her husband is James Shannon (b abt 1836). The census record suggests Margaret is from Limerick so her brother Denis must be as well.
Is it possible to find out where in Limerick they are from?
Can't see a marriage record for Denis and his sister.
All Denis' kids are born in London.