If you have an IRA with a brokerage account you can invest in crypto-related companies through the stock market. Nvidia, Overstock, and Bitcoin Investment Trust are a few traded on the NYSE or OTC markets. Sure, you're limited to $5500 a year in contributions, but you don't pay gains. I do this with my actual retirement account, which is about 50/50 with my cypto portfolio after the Jan 2018 drawdown.

Other speculative crypto investments in my portfolio include MGTI and Hive Blockchain. I have no plans to pull any money out of this fund until retirement.