Since 1903 the County Louth Archeaological and Historical Society has been encouraging interest in and research about County Louth’s past.  They publish a Journal as well.  It’s quite a useful resource for people (*ahem!*) working on historical fiction set in the area.

My understanding is that most if not all counties have some sort of historical society, and certainly a local museum, so if your research happens to take you to County Cork or County Mead rather than County Louth, most likely you’ll find a local historical society there as well.