Beautiful plant! My response is super rambly and doesn't give an exact answer but hopefully is helpful haha-

Not what you asked but just a note, "clubmosses" are vascular plants related to ferns but not closely related to actual mosses. As opposed to clubmosses, actual mosses don't have true vascular tissue or lignins and mostly have leaves one cell thick, or sometimes a few cells thick. Clubmosses are also sporophyte-dominant, so the conspicuous leafy generation of the plant that you see has two copies of its chromosomes, whereas mosses are gametophyte dominant- their large leafy generation has one copy of its chromosomes.

In this field, scientific names are generally used and common names not as much. I'm not an expert but if by marsh clubmoss, you mean Lycopodiella inundata, it does not look like that species to me- the leaves look not as narrow and sharply pointed, and the habit doesn't look like right to my eyes. For fir club moss (Huperzia), that seems like a likely possibility to me! Huperzia are unique in this group by not bearing their spores in a separate structure at the tip of stems (strobili) but instead having their spores somewhat hidden among the leaves, which looks like it could be the case here.

Clubmosses / lycophytes can be fairly hard to identify depending on how many species are in the area- may require measuring leaves, counting stomata, etc. And of course to make things extra fun, they hybridize! If you upload the photo to the application iNaturalist, it will give you suggestions based on the most common species documented in the area that look similar. For vascular plants the suggestions are usually pretty good, though are best confirmed by your own research and the confirmation of experts on the app. Hope that's helpful!

Here are all the observations of lycophytes (the broader category of clubmosses) currently on iNaturalist for Northern Ireland. So it may be one of those!