First, if you commit murder, you can still ask for forgiveness and be accepted into heaven. If you commit suicide, you don't have the opportunity to do that. So you end up in hell.
Second, homicide isn't a sin,
murder is. There's a difference.
Most commonly know of the commandment, "Thou shalt not kill". The actual commandment though, is "thou shall not
murder". Which changes things dramatically.
Homicide is just "one human killing another", but murder is more than that. Murder is
unjustly killing. And according to a religious government such as Ireland (or literally any other one you can think of), the law is as much a tool of God as it is a tool of man. In other words, if we condemn a man to die here for breaking our laws, then God holds that the same in heaven. This is derived from Matthew 16:19: "I will give you the keys of the kingdom of heaven; whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven." In other words, "If your authority holds it, then I will too".
So being condemned to death was never seen as anyone murdering anyone else. And similarly, we do the same thing today: The government doesn't consider capital punishment "murder" at all.
Are there logical holes to be seen here? Yes. But this is a practical reading of the logic of the time that they were using.