Here grandfather is called "sheanair", but everywhere else I look on the internet it says "seanair" so which one is correct ? Also here it sounds (when they read it our loud) like "henath" but on the internet it aounds more like "šenar" so which one is it ? Or am I missing something?
It's a little like the French ''vous", in that it's used for plural and/or formal. So you use leibh when thanking a group (whether formally or informally) or when thanking someone who you would speak to in a more respectful, formal way. Whether it is the case that you would always use leibh with your grandfather would depend I suspect on your relationship with them, but basically it's safer to use leibh with strangers, groups, and people higher than you on the status chain. So leat with brother or sister, but leibh for parents and grandparents. You may switch how you use it with those family members, depending on circumstance. I know that even in English people may vary whether they use mother, or mom/mum/ma depending on whether the conversation is more formal or informal.