Poignant and piercing, the duets of Martin Hayes and Dennis Cahill rely on the kind of understatement that ultimately amplifies the wistful glee often at the heart of trad Celtic music. It’s a big deal when the violinist and guitarist make their semi-annual New York stops – fans can’t wait to see them work their deep improv skills on items such as “Gallagher’s Frolics” or “Green Gowned Lass.” Guess that makes their two-week residency at the Irish Arts Center a mega-event. It centers on the Martin Hayes Quartet (the pair joined by violist Liz Knowles and bass clarinetist Doug Wieselman) while also boasting collabos with an array of regional heroes, expert in everything from mandolin to tin whistle. Perhaps most intriguing is the closing concert with jazz singer Cassandra Wilson, who named one of her discs ‘Glamoured,’ harking to the Gaelic definition of spells being cast and enchantments being rendered. Given the seductive rapport Hayes and Cahill have conjured previously, it’s an apt parallel to this string of exploratory shows.