It's simple noted.

If you say "irassyaimase", it's more polite. This term is normally used at stores, restaurants etc..."Irassyai" itself is more casual way of saying welcome. "Youkoso" is also the right word and used for morelikely for amusument park sorta places. So if you're selling something on your web site, "Irassyaimase" would be good. Otherwise, "youkoso" or "irassyai" is the word!
Try "(your name etc) no SITE ni YOUKOSO! (or IRASSYAI!)" =)

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For this context, I think either 歓迎 or ようこそ will be appropriate. As a Japanese native, I think ようこそ sound would sound the most natural and welcomed, if I were to visit the site. 歓迎is not inappropriate, it is more often used for the tourist industry. If I were you, I would use ようこそ！ as translation for "Welcome."