How to you say:

"I find it interesting"

?

Can you use "is" in conjunction with an adjective to make a verb?:

Is suimiúil liom é.

Say anything you want as long as you don't say "faighim suimiúil é" because that would be an anglicism :-)

I would probably reword it completely and say something less compact, like "ceapaim go bhfuil sé suimiúil".

I must say that "is suimiúil liom é" sounds very good. I don't think I've ever heard it but it sounds very native-like. You've obviously modelled it after "is maith liom é", "is ait liom é", and the like, and those are very nice and rounded expressions.

On the other hand -- and this is just my personal observation, please correct me of know otherwise -- it seems to me that fluent native speakers aren't too fond of evaluating things as "interesting", they prefer to say something more committal and more specific, like "I like it" or "I don't like it".

Comhthéacs? Is minic a úsáidtear "I find it interesting" le blaisín na híoróine. Mar shampla, I find it interesting that X is asking for international cooperation while at the same time.... = Is suimiúil (iontach, aisteach, etc.) an rud é go bhfuil X ag iarraidh...

Context:

I went back the next day because I found it interesting.

-Did you read his philosophy?
-Yes.
-Did you find it interesting?

Chuaigh mé ar ais an chéad lá eile toisc go raibh suim agam ann.

An raibh sé suimiúil?
An raibh suim/spéis agat ann?
Ar mhúscail sé suim/spéis ionat?