WEYMOUTH — Waving a shark puppet in front of a window for a squealing toddler is a far cry from playing hard rock to a mosh pit on St. Patrick's Day weekend, but that's where the Dropkick Murphys found themselves this week.

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The members of Dropkick Murphys, a American Celtic punk band that formed in Quincy in the late 1990s, posted a video to their Facebook page Thursday of a private concert two members gave for Quinn Waters, a 3-year-old with brain cancer who has captured many hearts across the South Shore in recent months. Quinn, a Weymouth resident and son of a Quincy police officer, is currently fighting medulloblastoma, an aggressive brain tumor.

"Quinn has a wonderful family and they are in our thoughts and prayers during this difficult time," the band posted to Facebook.

Quinn, who has been dubbed "The Mighty Quinn" by supporters, recently underwent surgery to have most of the tumor removed and has been through numerous rounds of chemotherapy, months of hospitalizations and a stem cell transplant. He is in isolation at home, where he has been holding court for hundreds of well-wishers who communicate with him through the front window of his house.

On Wednesday, Dropkick lead singer and Milton native Ken Casey and guitarist Tim Brennan dropped by to perform a few songs for the Waters family, including the popular "Shipping Up to Boston." In the video, Casey can be seen singing to the excited toddler with a shark puppet on his hand, and Quinn can be heard squealing in the background.

"We cannot be more thankful to everyone, but especially today, to Ken Casey and Tim Brennan for taking the time to come see Quinn," the Waters family posted to The Mighty Quinn Facebook page.

The family also thanked Casey for the support of his charity, the Claddagh Fund, which raises money to support "vulnerable populations in our communities."

"Thanks also to Ken and the Claddagh Fund — since the day we found out Quinn's diagnosis they were right there offering to help any way that they could," Tara Waters, Quinn's mom, wrote. "Quinn will most certainly be rocking his DKM gear at his next clinic visit!"