In the 'search this forum' box type "How do Glackens", then click 'Go'.This should return a thread of postings that gives some background on the various spellings of the name, including reference to the Mac/Mc prefix. You might find this to be a useful starting place.You might also find a posting from Paul Glackin to be informative (just do a search on his name).
But! Once you've familiarized yourself with the variations of the Glackin name, don't think you'll find much more help on McGlackin in this forum.If I were you, I'd use the 'Add Forum' option to start one that's very specific to the McGlackin name (doesn't look like one already exists).Then I'd post a message looking for other McGlackens and wait for others with your surname to find it and start posting to it.(If you build it, they will come).
And don't forget to check back to the Glackin forum periodically since it's certainly a variation of the same name.Good Luck!