Yes, you have to know the name of the tune to search for it. The site knows nothing about who performs it. And some tunes have several names, and the name on a Clannad album might not be the most common name.

As an experiment, I looked up Clannad albums in Discogs then searched for all the tunes on the first album.

Níl Sé Ina Lá
Thíos Cois Na Trá Domh
Brian Boru's March
Siúbhán Ní Dhuibhir
An Mhaighdean Mhara
Liza
An TOileán Úr
Mrs. McDermott (Magistreas Mc Diarmuid)
The Pretty Maid (An Cailín Deas)
An Pháirc
Harvest Home
Morning Dew

Some of the ones I found have slightly different names. But I'd have to know the album to know whether the tunes are the same.

For example, I search for 'Mrs. McDermott' - result.
The first result gives Magistreas Mc Diarmuid. I do not know Irish but I assume 'Magistreas' might mean 'Mistress' which is what 'Mrs' is short for.

So I click the green 'tune page': Magistreas Mc Diarmuid and play it using the ? button. If that is not the one I want I try the next result. It can take a long time and there is no guarantee of success.

(And I often get diverted by what I find and end up with a different tune altogether. It's a huge time waster - but very rewarding.)

You can use the strange diagram on the right to find similar tunes, maybe one with chords or in a better key for you.

That's the way I have been using the site. There are probably better ways, and I know there are other tune finding sites.