Here's a discussion group page on the tradconnect website, "connecting traditional Irish musicians across the world," at http://tradconnect.com/group/the-hammered-dulcimer-grou-irish-traditional-music. Says Dick Glasgow of Northern Ireland, "The Hammered Dulcimer has been here & playing Irish Music in Ireland at least since 1738, when we know one Archibald Williamson, was active in Dublin."
And an interesting thread on The Session, at https://thesession.org/discussions/25484", concluding -- but not quite conclusively -- that "as voiced by many session musicians over the years, it is not what you might call the most popular of instruments, when it appears in the Pub doorway, under someone’s arm."
(A sentiment that mountain dulcimer people also run into.)
Dick Glasgow's website https://dickglasgow.wordpress.com/about/
Standard reference is David Kettlewell's thesis at ____________ University in England. Summarized on line at Dulcimer-players and Their Instruments. http://dulcimer.new-renaissance.com/ ["David Kettlewell's doctoral thesis 'The Dulcimer' (1976 with updates) is gradually being presented here"]
"William Carleton & the William Carleton Summer School." William Carleton Summer School, Corick House Hotel, Clogher, County Tyrone, 2-5 August 2010. http://www.dungannon.gov.uk/pubuploads/William%20Carleton%20Summer%20School%20Leaflet.pdf