As you can imagine, legal disputes over festival names are not uncommon (even when the festivals offer different services). But to stop someone from using a similar festival title, you need more than a domain name; you need trademark rights. You receive those rights by being the first to use the name in commerce. In other words, if you used Houston Reggae Festival in commerce starting in May 2015, you might have superior rights (referred to as "priority") over subsequent users.
What is "use in commerce?"Your mark is "in use in commerce" when the services are actually advertised, and you can legitimately deliver festival services to customers. Actual use must be a commercial use, not simply a token use to obtain trademark priority. So, if you promote a festival but you cannot provide festival entertainment, your mark is not in use.
Should you apply for federal registration? We believe Houston Reggae Festival is registrable. You will need to disclaim "Reggae Festival" (but not Houston), and you should register in Class 041. That's based on two existing trademark registrations: Kentucky Reggae Festival (Reg. No. 2292636) and Charlemont Reggae Festival (Reg. No. 88104947).
Registration complications. Although the mark is not generic, a USPTO trademark examiner will likely object to it as being "descriptive." To overcome this and achieve registration, you will need to demonstrate that your mark has become distinctive for reggae festivals. You prove this by demonstrating substantial marketing and advertising, or by five years of continuous use. If the examiner classifies your mark as descriptive and you can't prove distinctiveness, you may need to switch your application to the Supplemental Register. Also, you cannot file an ITU application for a descriptive mark.
What should you do? There are too many variables in play -- your date of first use, whether the mark is descriptive, whether you have the financial support needed to take on a trademark dispute -- that we cannot plot your next move. Surely, if you're hosting or planning to host music festivals, you should resolve your name dispute, and apply for trademark registration. As to the order of events, you'd be best served by consulting an attorney. Consider contacting Texas Accountants and Lawyers for the Arts.