Why are the Irish slaves forgotten? Were they even slaves or were they indentured servants?

The Irish were not slaves in the chattel sense. This means that their offspring were not sold. Many Irish came to the new world as indentured servants in order to find a new life in America. Since they could not afford the ticket, many worked off their passage for approximately seven years, after which they were free to pursue their own lives. While some who used indentured servants were cruel and prolonged the contract, this is not the same as slavery. Many Irish would die as a result of the bad conditions of the New World such as bad food and water leading to disease, but mortality among the poor of Europe was still high, so one cannot argue that the indentured servants were intentionally placed in harm's way.
The British would also send Irish political prisoners to work in the Caribbean sugar plantations. This would serve as a virtual death sentence since conditions on the islands were extreme in terms of the diseases one could catch. This was considered punishment more than a way to solve a labor shortage. Britain still relied on African slaves for its manual labor needs in the colonies.