As the Devlins are an Irish clan/family they do not have a family "tartan" as such. However, this does not mean that there are no tartans which are appropriate.
Recently, a range of tartans was launched for the thirty two counties of Ireland (including the north). As the O'Devlin clan had its lands in Tyrone, then that is perhaps the most appropriate from the range. However, the colours are a little dull, a mixture of maroon and light brown, green etc. Alternatively, if you can place your Devlin family in a county in Ireland before they left (assuming they have !), then this would be another choice.
However, when I ordered a Kilt recently, I chose one of the two Ulster tartans, the Ulster Red (Forest green/bright red and gold). (The other is the more common Ulster saffron, which is yellow/gold/light brown with some red/orange). These tartans have been around for a lot longer than the Irish county range and have more historical validity.
It should also be remembered that Ulster is essentially a geographical term, rather than a political one. As I'm sure you know, the historical province of Ulster contains not only the the six counties, but also Cavan, Donegal and Monaghan. So its suitable for all individuals with Ulster roots, regardless of political persuasion.
I have gif/bmp files with samples of the tartans as well as web site addresses that could help you. Please e mail me if you need any further information.